Ductile stainless steel pipe and heat pump system comprising the same

ABSTRACT

Provided is a ductile stainless steel pipe made of stainless steel having an austenite type matrix structure and containing a copper component. The ductile stainless steel pipe has a delta ferrite matrix structure of about 1% or less on the basis of a grain area. The ductile stainless steel pipe includes a steel pipe having a set outer diameter to carry a refrigerant of an air conditioner. R410a is used as the refrigerant, and the ductile stainless steel pipe has a minimum thickness determined based on a saturated pressure of the refrigerant.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean patent application No.10-2017-0042164, filed on Mar. 31, 2017, the entire content of which isincorporated herein by reference.

BACKGROUND

The present disclosure relates to a ductile stainless steel pipe and toa heat pump system comprising the same.

Air conditioners may be defined as devices for supplying warm air orcold air to an indoor space by using a phase change cycle of arefrigerant.

In detail, the phase change cycle of the refrigerant may include acompressor compressing a low-temperature low-pressure gas refrigerant tochange into a high-temperature high-pressure gas refrigerant, acondenser allowing the high-temperature high-pressure gas refrigerantcompressed in the compressor to phase-change into a high-temperaturehigh-pressure liquid refrigerant, an expansion valve expanding thehigh-temperature high-pressure liquid refrigerant passing through thecondenser to change into a low-temperature low-pressure two-phaserefrigerant, and an evaporator allowing the low-temperature low-pressuretwo-phase refrigerant passing through the expansion valve tophase-change into a low-temperature low-pressure gas refrigerant.

When the phase change cycle of the refrigerant operates as a device forsupplying cold air, the condenser is disposed in an outdoor space, andthe evaporator is disposed in an indoor space. Also, the compressor, thecondenser, the expansion valve, and the evaporator are connected to eachother through a refrigerant pipe to form a closed refrigerantcirculation loop.

In general, a copper (Cu) pipe made of a copper material is widely usedas the refrigerant pipe. However, the copper pipe has some limitationsas follows.

First, when the copper pipe is used in a total heat exchanger in whichwater is used as a refrigerant, scales are accumulated on an innercircumferential surface of the pipe to deteriorate reliability of thepipe. That is, when the scales are accumulated on the innercircumferential surface of the copper pipe, it is necessary to perform acleaning process for cleaning the inner circumferential surface of thepipe or a pipe replacement process.

Second, there is a disadvantage that the copper pipe does not havesufficient pressure resistance characteristics for withstanding a highpressure. Particularly, when the copper pipe is applied to a refrigerantcirculation cycle to which a refrigerant compressed at a high pressureby a compressor, i.e., a new refrigerant such as R410a, R22, and R32 isapplied, as an operating time of the refrigerant cycle is accumulated,the cooper pipe may not withstand the high pressure and thus be damaged.

Third, since the copper pipe has a small stress margin value forwithstanding a pressure of the refrigerant in the pipe, it is vulnerableto vibration transmitted from the compressor. For this reason, to absorbthe vibration transmitted to the copper pipe and the resultant noise,the pipe is lengthened in length and disposed to be bent in x, y, and zaxis directions.

As a result, since an installation space for accommodating the copperpipe is not sufficient in an outdoor unit of an air conditioner or awashing machine using a heat pump, it is difficult to install the pipe.

Also, since copper prices are relatively high in the market, and pricefluctuations are so severe, it is difficult to use the copper pipe.

In recent years, to solve these limitations, a new method for replacingthe copper pipe with a stainless steel pipe is emerging.

The stainless steel pipe is made of a stainless steel material, hasstrong corrosion resistance when compared to the copper pipe, and isless expensive than that of the copper pipe. Also, since the stainlesssteel pipe has strength and hardness greater than those of the copperpipe, vibration and noise absorption capability may be superior to thatof the copper pipe.

Also, since the stainless steel pipe has pressure resistancecharacteristics superior to those of the copper pipe, there is no riskof damage even at the high pressure.

However, since the stainless steel pipe according to the related art hasexcessively high strength and hardness when compared to the copper pipe,it is disadvantageous to an expansion operation for pipe connection or apipe bending operation. Particularly, the pipe constituting therefrigerant cycle may be disposed in a shape that is bent at a specificcurvature at a specific point. However, when the stainless steel pipeaccording to the related art is used, it is impossible to bend the pipe.

SUMMARY

Embodiments provide a ductile stainless steel pipe which is improved inworkability by securing ductility at a level of a copper pipe.

Embodiments also provide a ductile stainless steel pipe having strengthand hardness equal to or higher than those of a copper pipe.

Embodiments also provide a ductile stainless steel pipe which is capableof preventing the pipe from corroded by a refrigerant pressure conditioninside the pipe or an environmental condition outside the pipe.

Embodiments also provide a ductile stainless steel pipe which is capableof maintaining a critical pressure above a predetermined level even ifthe pipe is reduced in thickness.

Embodiments also provide a ductile stainless steel pipe which increasesin inner diameter to reduce a pressure loss of a refrigerant flowing inthe pipe.

Embodiments also provide a ductile stainless steel pipe which isimproved in vibration absorption capability. Embodiments also provide aductile stainless steel pipe which is capable of effectively absorbingvibration transmitted from a compressor even if the pipe is shortened inlength.

The object is solved by the features of the independent claims.Preferred embodiments are given in the dependent claims.

In one embodiment, a ductile stainless steel pipe is made of stainlesssteel having an austenite type matrix structure and contains a coppercomponent, wherein the ductile stainless steel pipe has a delta ferritematrix structure of about 1% or less on the basis of a grain area, theductile stainless steel pipe includes a steel pipe having a set outerdiameter to carry a refrigerant of an air conditioner, R410a is used asthe refrigerant, and the ductile stainless steel pipe has a minimumthickness determined based on a saturated pressure of the refrigerant.

The ductile stainless steel pipe having at least a portion having anaustenite type matrix structure and contains a copper component, whereinthe ductile stainless steel pipe has a delta ferrite matrix structure ofabout 1% or less. I.e. at least a part of the ductile stainless steelpipe has the inventive composition. So according to the invention it isnot required, but possible, to provide the complete ductile stainlesssteel pipe with the austenite type matrix structure including copper andthe delta ferrite matrix structure of about 1% or less on the basis of agrain area.

Preferably, the steel pipe has an outer diameter of about 7.00 mm and aminimum thickness of about 0.18 mm.

Preferably, the steel pipe has an outer diameter of about 7.94 mm and aminimum thickness of about 0.20 mm.

Preferably, the steel pipe has an outer diameter of about 9.52 mm and aminimum thickness of about 0.24 mm.

Preferably, the steel pipe has an outer diameter of about 12.70 mm and aminimum thickness of about 0.33 mm.

Preferably, the steel pipe has an outer diameter of about 15.88 mm and aminimum thickness of about 0.41 mm.

Preferably, the steel pipe has an outer diameter of about 19.05 mm and aminimum thickness of about 0.49 mm.

Preferably, the steel pipe has an outer diameter of about 22.20 mm and aminimum thickness of about 0.57 mm.

Preferably, the steel pipe has an outer diameter of about 22.40 mm and aminimum thickness of about 0.65 mm.

Preferably, the steel pipe has an outer diameter of about 28.00 mm and aminimum thickness of about 0.72 mm.

Preferably, the steel pipe has an outer diameter of about 31.80 mm and aminimum thickness of about 0.81 mm.

Preferably, the steel pipe has an outer diameter of about 34.90 mm and aminimum thickness of about 0.89 mm.

Preferably, the steel pipe has an outer diameter of about 38.10 mm and aminimum thickness of about 0.98 mm.

The object is also solved by a heat pump system having an outdoor unitand an indoor unit, wherein the outdoor unit comprises a ductilestainless steel pipe as defined above.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features will be apparent fromthe description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a refrigeration cycle diagram of an air conditioner to which aductile stainless steel pipe is applied according to an embodiment.

FIG. 2 is a view illustrating a suction pipe and a discharge pipe of acompressor to which the ductile stainless steel pipe is appliedaccording to an embodiment.

FIG. 3 is a microstructure photograph of a stainless steel having anaustenite matrix structure of about 99% and a delta ferrite structure ofabout 1% or less.

FIG. 4 is a microstructure photograph of a stainless steel having onlythe austenite matrix structure.

FIG. 5 is a view illustrating an outer diameter and an inner diameter ofa refrigerant pipe according to an embodiment.

FIG. 6 is a flowchart illustrating a method for manufacturing theductile stainless steel pipe according to an embodiment.

FIG. 7 is a schematic view of a cold rolling process of FIG. 6.

FIG. 8 is a schematic view of a slitting process of FIG. 6.

FIG. 9 is a schematic view of a forming process of FIG. 6.

FIGS. 10 to 13 are cross-sectional views illustrating a process ofmanufacturing a ductile stainless steel pipe according to themanufacturing method of FIG. 6.

FIG. 14 is a schematic view of a bright annealing process of FIG. 6.

FIG. 15 is a graph illustrating result values obtained through an S-Ncurve test for comparing fatigue limits of the ductile stainless steelpipe according to an embodiment and a copper pipe according to therelated art.

FIG. 16 is a graph illustrating an S-N curve teat of the ductilestainless steel pipe according to an embodiment.

FIG. 17 is a view illustrating an attachment position of a stressmeasurement sensor for measuring stress of the pipe.

FIGS. 18 and 19 are test data tables illustrating result values measuredby the stress measurement sensor of FIG. 17.

FIG. 20 is a graph illustrating result values obtained through a testfor comparing pressure losses within the pipes when each of the ductilestainless steel pipe according to an embodiment and the copper pipeaccording to the related art is used as a gas pipe.

FIG. 21 is a test result table illustrating performance of the ductilestainless steel pipe according to an embodiment and the copper pipeaccording to the related art.

FIG. 22 is a view illustrating a plurality of ductile stainless steelpipes, aluminum (Al) pipes, and copper pipes, which are objects to betested for corrosion resistance.

FIG. 23 is a table illustrating results obtained by measuring acorrosion depth for each pipe in FIG. 22.

FIG. 24 is a graph illustrating results of FIG. 23.

FIG. 25 is view illustrating a shape in which the ductile stainlesssteel pipe is bent according to an embodiment.

FIG. 26 is a cross-sectional view illustrating a portion of the bentpipe.

FIG. 27 is a graph illustrating results obtained through a test forcomparing bending loads according to deformation lengths of the ductilestainless steel pipe, the copper pipe, and the aluminum pipe.

FIG. 28 is a refrigeration cycle diagram of an air conditioner to whicha ductile stainless steel pipe is applied according to anotherembodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments of the present disclosure will be described belowin more detail with reference to the accompanying drawings. It is notedthat the same or similar components in the drawings are designated bythe same reference numerals as far as possible even if they are shown indifferent drawings. In the following description of the presentdisclosure, a detailed description of known functions and configurationsincorporated herein will be omitted to avoid making the subject matterof the present disclosure unclear.

In the description of the elements of the present disclosure, the terms“first”, “second”, “A”, “B”, “(a)”, and “(b)” may be used. However,since the terms are used only to distinguish an element from another,the essence, sequence, and order of the elements are not limited bythem. When it is described that an element is “coupled to”, “engagedwith”, or “connected to” another element, it should be understood thatthe element may be directly coupled or connected to the other elementbut still another element may be “coupled to”, “engaged with”, or“connected to” the other element between them.

FIG. 1 is a refrigeration cycle diagram of an air conditioner to which aductile stainless steel pipe is applied according to an embodiment, andFIG. 2 is a view illustrating a suction pipe and a discharge pipe of acompressor to which the ductile stainless steel pipe is appliedaccording to an embodiment.

<Configuration of Outdoor Unit>

Referring to FIG. 1, an air conditioner 10 according to an embodimentinclude an outdoor unit 20 and an indoor unit 160 to operate arefrigerant cycle in which a refrigerant circulates. First, aconfiguration of the outdoor unit 20 will be described.

[Compressor]

Referring to FIG. 1, the air conditioner to which a ductile stainlesssteel pipe is applied according to an embodiment includes a compressor100 compressing the refrigerant.

Refrigeration capacity, i.e., air-conditioning capacity of the airconditioner 10 may be determined based on compressibility of thecompressor 100. The air-conditioning capacity may include coolingcapacity or heating capacity. The air conditioner 10 according to thisembodiment may have air-conditioning capacity of about 7 kW to about 8kW.

The compressor 100 includes a rotary-type inverter compressor. Forexample, the compressor 100 includes a twin rotary compressor. Also, alimit refrigerant amount of compressor 100 may about 2,500 cc, and anamount of used oil of the compressor 100 is about 900 cc.

Here, the compressor 100 is not limited to the rotary-type invertercompressor, e.g., may include one of a scroll compressor, areciprocating compressor, and a linear compressor.

[Muffler]

The air conditioner 10 further includes a muffler 105 disposed at anoutlet side of the compressor 100. The muffler 105 may reduce noisegenerated from a high-pressure refrigerant discharged from thecompressor 100. The muffler 105 includes a chamber for increasing a flowcross-sectional area of the refrigerant, and the chamber defines aresonance chamber.

[Flow Control Valve]

The air conditioner 10 further include a flow control valve 110 disposedat an outlet side of the muffler 105 to convert a flow direction of therefrigerant compressed in the compressor 100.

For example, the flow control valve 110 may include a four-way valve. Indetail, the flow control valve 110 includes a plurality of ports. Theplurality of ports include a first port 111 into which a high-pressurerefrigerant compressed in the compressor 100 is introduced, a secondport 112 connected to a pipe extending from the flow control valve 110to an outdoor heat exchanger, a third port 113 connected to a pipeextending from the flow control valve 110 to the indoor unit 160, and afourth port 114 extending from the flow control valve 110 to agas/liquid separator 150.

[Operation of Flow Control Valve During Cooling/Heating Operation]

The refrigerant compressed in the compressor 100 may pass through themuffler 105 and then be introduced into the flow control valve 110through the first port 111 of the flow control valve 110.

When the air conditioner 10 performs a cooling operation, therefrigerant introduced into the flow control valve 110 may flow to theoutdoor heat exchanger 120. For example, the refrigerant may bedischarged from the second port 112 of the flow control valve 110 andthen introduced into the outdoor heat exchanger 120.

On the other hand, when the air conditioner 10 performs a heatingoperation, the refrigerant introduced into the flow control valve 110may flow to the indoor unit 160. For example, the refrigerant may bedischarged from the third port 113 of the flow control valve 110 andthen introduced to the indoor unit 160.

[Outdoor Heat Exchanger and Outdoor Fan]

The air conditioner 10 further includes an outdoor heat exchanger 120heat-exchanged with external air. The outdoor heat exchanger 120 isdisposed at an outlet side of the flow control valve 110.

The outdoor heat exchanger 120 further includes a heat exchange pipe 121and a holder 123 supporting the heat exchange pipe 121. The holder 123may support both sides of the heat exchange pipe 121. Although not shownin the drawings, the outdoor heat exchanger 120 further includes a heatexchange pin coupled to the heat exchange pipe 121 to assist theheat-exchange with the external air.

An outdoor fan 125 blowing the external air to the outdoor heatexchanger 120 is further provided at one side of the outdoor heatexchanger 120.

[Manifold and Connection Pipe]

The air conditioner 10 further includes a manifold 130 connected to thefirst port of the flow control valve 110. The manifold 130 is disposedat one side of the outdoor heat exchanger 120 and configured to allowthe refrigerant to be introduced into a plurality of passages of theoutdoor heat exchangers 120 during the cooling operation and allow therefrigerant passing through the outdoor heat exchanger 120 to becollected.

The air conditioner 10 includes a plurality of connection pipes 135extending from the manifold 130 to the outdoor heat exchanger 120. Theplurality of connection pipes 135 may be disposed spaced apart from eachother from an upper portion to a lower portion of the manifold 130.

[Distributor]

A distributor 140 is disposed at one side of the outdoor heat exchanger120. The distributor 140 may be configured to mix the refrigerantpassing through the outdoor heat exchanger 120 during the coolingoperation or distribute and introduce the refrigerant into the outdoorheat exchanger 120 during the heating operation.

[Capillary and Branch Pipe]

The air conditioner 10 further includes a plurality of capillaries 142extending from the distributor 140 to the outdoor heat exchanger 120.Each of the capillaries 142 may be connected to the branch pipe 145. Thebranch pipe 145 may be coupled to the outdoor heat exchanger 120. Forexample, the branch pipe 145 may have a Y shape and be coupled to a heatexchange pipe 121 of the outdoor heat exchanger 120. A plurality ofbranch pipes 145 may be provided to correspond to the plurality ofcapillaries 142.

[Expansion Device and Strainer]

The air conditioner 10 further includes a main expansion device 155decompressing the refrigerant condensed in the indoor unit 160. Forexample, the main expansion device 155 may include an electronicexpansion valve (EEV) of which an opening degree is adjustable.

Strainers 156 and 158 separating foreign substances from the refrigerantare further provided at one side of the expansion device 155. Thestrainers 156 and 158 may be provided in plurality. The plurality ofstrainers 156 and 158 may include a first strainer 156 disposed at oneside of the expansion device 155 and a second strainer 158 disposed atthe other side of the expansion device 155.

When the cooling operation is performed, the refrigerant condensed inthe outdoor heat exchanger 120 may pass through the first strainer 156and then pass through the second strainer 158 via the expansion device155. On the other hand, when the heating operation is performed, therefrigerant condensed in the indoor unit 160 may pass through the secondstrainer 158 and then pass through the first strainer 156 via theexpansion device 155.

[Service Valve and Installation Pipe]

The outdoor unit 20 further includes service valves 175 and 176connected to the connection pipes 171 and 172 when being assembled withthe indoor unit 160. The connection pipes 171 and 172 may be understoodas pipes connecting the outdoor unit 20 to the indoor unit 160.

The service valves 175 and 176 include a first service valve 175disposed in one portion of the outdoor unit 20 and a second servicevalve 176 disposed in the other portion of the outdoor unit 20.

Also, the connection pipes 171 and 172 include a first connection pipe171 extending from the first service valve 175 to the indoor unit 160and a second connection pipe 172 extending from the second service valve176 to the indoor unit 160. For example, the first connection pipe 171may be connected to one side of the indoor unit 160, and the secondconnection pipe 172 may be connected to the other side of the indoorunit 160.

[Pressure Sensor]

The outdoor unit 20 further includes a pressure sensor 180. The pressuresensor 180 may be installed in the refrigerant pipe extending from thethird port 113 of the flow control valve 110 to the second service valve176.

When the cooling operation is performed, the pressure sensor 180 maydetect a pressure, i.e., a low pressure of the refrigerant evaporated inthe indoor unit 160. On the other hand, the pressure sensor 180 maydetect a pressure, i.e., a high pressure of the refrigerant compressedin the compressor 100.

[Gas/Liquid Separator]

The outdoor unit 20 further includes a gas/liquid separator 150 disposedat a suction side of the compressor 100 to separate a gas refrigerant ofthe evaporated low-pressure refrigerant and thereby supply the separatedrefrigerant to the compressor 100. The gas/liquid separator 150 may beinstalled in the suction pipe 210.

The suction pipe 210 extends from the fourth port 114 of the flowcontrol valve 110 to the compressor 100. The gas refrigerant separatedby the gas/liquid separator 150 may be suctioned into the compressor100.

<Configuration of Indoor Unit>

The indoor unit 160 includes an indoor heat exchanger (not shown) and anindoor fan disposed on one side of the indoor heat exchanger to blowindoor air. Also, the indoor unit 160 may further include an indoorexpansion device decompressing the condensed refrigerant when thecooling operation is performed. Also, the refrigerant decompressed inthe indoor expansion device may be evaporated in the indoor heatexchanger.

The indoor unit 160 may be connected to the outdoor unit 20 through thefirst and second connection pipes 171 and 172.

[Refrigerant Pipe]

A plurality of constituents of the outdoor unit 20 may be connected tothe indoor unit 160 through the refrigerant pipe 50, and the refrigerantpipe 50 may guide refrigerant circulation in the outdoor unit 20 and theindoor unit 160. The first and second connection pipes 171 and 172 mayalso be understood as one component of the refrigerant pipe 50.

A pipe diameter (an outer diameter) of the refrigerant pipe 50 may bedetermined based on air-conditioning capacity of the air conditioner 10.For example, when the air-conditioning capacity of the air conditioner10 increases, the pipe diameter of the refrigerant pipe 50 may bedesigned to be relatively large.

[Refrigerant Flow During Cooling Operation]

When the air conditioner 10 performs the cooling operation, therefrigerant compressed in the compressor 100 is introduced into thefirst port 111 of the flow control valve 110 via the muffler 105 andthen discharged through the second port 112. The refrigerant dischargedfrom the flow control valve 110 is introduced into the outdoor heatexchanger 120 and then condensed to pass through the main expansiondevice 155 via the distributor 140 and the first strainer 156. Here, thedecompression of the refrigerant does not occur.

Also, the decompressed refrigerant is discharged from the outdoor unit20 after passing through the second strainer 158. Then, the refrigerantis introduced into the indoor unit 160 through the first connection pipe171 and decompressed in the indoor expansion device and then evaporatedin the indoor heat exchanger of the indoor unit 160. The evaporatedrefrigerant is introduced again into the outdoor unit 20 through thesecond connection pipe 172.

The refrigerant introduced into the outdoor unit 20 is introduced intothe flow control valve 110 through the third port 113 and dischargedfrom the flow control valve 110 through the fourth port 114. Also, therefrigerant discharged from the flow control valve 110 isphase-separated in the gas/liquid separator 150, and the separated gasrefrigerant is suctioned into the compressor 100.

This cycle may be repeatedly performed.

[Refrigerant Flow During Heating Operation]

When the air conditioner 10 performs the heating operation, therefrigerant compressed in the compressor 100 is introduced into thefirst port 111 of the flow control valve 110 via the muffler 105 andthen discharged through the third port 113. The refrigerant dischargedfrom the flow control valve 110 is introduced into the indoor unit 160through the second connection pipe 172 and discharged from the indoorunit 160 after being condensed in the indoor heat exchanger. Therefrigerant discharged from the indoor unit 160 is introduced into theoutdoor unit 20 through the first connection pipe 171 and then isdecompressed in the main expansion device 155 via the second strainer158.

Also, the decompressed refrigerant is branched introduced into theoutdoor heat exchanger 120 through the distributor 140 and the capillary142 after passing through the first strainer 150. Then, the refrigerantis evaporated in the outdoor heat exchanger 120 and then is introducedinto the flow control valve 110 through the second port 112.

Also, the refrigerant is discharged from the flow control valve 110through the fourth port 114 and phase-separated in the gas/liquidseparator 150, and the separated gas refrigerant is suctioned into thecompressor 100. This cycle may be repeatedly performed.

[Refrigerant]

The refrigerant may circulate through the outdoor unit 20 and the indoorunit 160 to perform the cooling or heating operation of the airconditioner 10. For example, the refrigerant may include R21 or R134a asa single refrigerant.

The R32 is a methane-based halogenated carbon compound and expressed byFormula CH2F2. The R32 is an eco-friendly refrigerant having ozonedepletion potential (ODP) less than that of the R22 (Chemical Formula:CHCLF2) according to the related art, and thus, a discharge pressure ofthe compressor is high.

The R134a is an ethane-based halogenated carbon compound and expressedby Formula CF3CH2F. The R134a may be used for the air conditioner as arefrigerant replacing the R12 (Chemical Formula: CCl2F2) according tothe related art.

For another example, the refrigerant may include R410a as anon-azeotropic mixed refrigerant.

The R410a is a material in which the R32 and R125 (Chemical FormulaCHF₂CF₃) are mixed at a weight ratio of 50:50. When the refrigerant isevaporated (saturated liquid=>saturated gas) in the evaporator, atemperature increases, and when the refrigerant is condensed (saturatedgas=>saturated liquid) in the condenser, the temperature decreases. As aresult, heat exchange efficiency may be improved.

For another example, the refrigerant may include R407c as anon-azeotropic mixed refrigerant. The R407c is a material in which theR32, the R125, and the R134a are mixed at a weight ratio of 23:25:52.Since the R407c has ozone destruction coefficient less than that of theR22 according to the related art and a vapor pressure similar to that ofthe R22, the replacement of the equipment constituting the existingrefrigeration cycle may be minimized to reduce the cost.

In this embodiment, the R410a is used as the refrigerant circulatingthrough the air conditioner 10.

[Refrigerant Circulation Amount]

The refrigerant may be filled into the air conditioner 10 according toan embodiment. A filling amount of refrigerant may be determined basedon a length of the refrigerant pipe 50 constituting the air conditioner10. For example, about 2,000 g of the refrigerant may be filled based ona standard pipe having a length of about 7.5 m, and about 3,400 g of therefrigerant may be filled based on a long pipe having a length of about50 m. In addition, about 40 g of the refrigerant may be filled into anadditional pipe.

Also, the capacity of the refrigerant compressed in the compressor 100may be determined based on the air-conditioning capacity of the airconditioner 10. Like this embodiment, a limit amount of refrigerantwithin the compressor 100 may be about 2,500 cc on the based of theair-conditioning capacity of about 7 kW to about 8 kW.

[Oil]

Oil for lubricating or cooling the compressor 100 is contained in theair conditioner according to an embodiment. The oil may include aPAG-based refrigerating machine oil, a PVE-based refrigerating machineoil, or a POE-based refrigerating machine oil.

The PAG-based refrigerating machine oil is a synthetic oil made ofpropylene oxide as a raw material and has a relatively high viscosityand thus has excellent viscosity characteristics depending on atemperature. Thus, when the PAG-based refrigerating machine oil is used,the compressor may be reduced in load.

The PVE-based refrigerating machine oil is a synthetic oil made of vinylether as a raw material and has good compatibility with the refrigerant,high volume resistivity, and excellent electrical stability. Forexample, the PVE-based refrigerating machine oil may be used for thecompressor using the refrigerant such as the R32, the R124a, or theR410a.

The POE-based refrigerating machine oil is a synthetic oil obtained bydehydrating condensation of polyhydric alcohol and carboxylic acid andhas good compatibility with the refrigerant and also has excellentoxidation stability and thermal stability in air. For example, thePOE-based refrigerating machine oil may be used for the compressor usingthe refrigerant such as the R32 or the R410a.

In this embodiment, the PVE-based refrigerating machine oil, e.g.,FVC68D may be used as the refrigerating machine oil.

[New Material Pipe]: Ductile Stainless Steel Pipe

The refrigerant pipe 50 may include a new material pipe that is strongand having excellent workability. In detail, the new material pipe maybe made of a stainless steel material and a material having at leastcopper (Cu)-containing impurities. The new material pipe has strengthgreater than that of a copper (Cu) pipe and workability superior to thatof the stainless steel pipe. For example, the new material pipe may becalled a “ductile stainless steel pipe”. The ductile stainless steelpipe refers to a pipe made of ductile stainless steel.

When the refrigerant pipe 50 is provided as the copper pipe, a kind ofrefrigerant circulating through the copper pipe may be limited. Therefrigerant may be different in operation pressure range according tothe kind of refrigerant. If the high-pressure refrigerant having a highoperation pressure range, that is, a high pressure that is capable ofincreasing is used for the copper pipe, the copper pipe may be broken,and thus the leakage of the refrigerant may occur.

However, when the ductile stainless steel pipe is used as the newmaterial pipe like this embodiment, the above-described limitation maybe prevented from occurring.

[Property of Ductile Stainless Steel]

The ductile stainless steel has strength and hardness less than those ofthe stainless steel according to the related art, but has a good bendingproperty. The ductile stainless steel pipe according to an embodimenthas strength and hardness less than those of the stainless steelaccording to the related art, but remains to at least the strength andhardness of the copper pipe. In addition, since the ductile stainlesssteel pipe has a bending property similar to that of the copper pipe,bending workability may be very good. Here, the bending property and thebendability may be used in the same sense.

As a result, since the ductile stainless steel pipe has strength greaterthan that of the copper pipe, the possibility of the damage of the pipemay be reduced. Thus, there is an effect that the number of types ofrefrigerant capable of being selected in the air conditioner increases.

[Suction Pipe of Compressor]

The refrigerant pipe 50 includes a suction pipe 210 guiding suction ofthe refrigerant into the compressor 100. The suction pipe 210 may beunderstood as a pipe extending from the fourth port 114 of the flowcontrol valve 110 to the compressor 100.

The suction pipe 210 may include the ductile stainless steel pipe. Also,since a low-pressure gas refrigerant flows through the suction pipe 210,the suction pipe 210 may have a relatively large outer diameter of about15.80 mm to about 15.95 mm. When at least two pipes are connected toeach other, and then one pipe is expanded, the suction pipe 210 may havean outer diameter corresponding to an outer diameter of the expandedpipe.

For example, the suction pipe 210 may have an outer diameter of about15.88 mm. Also, the suction pipe 210 may have an inner diameter of about15.06 or less in consideration of a thickness of the suction pipe 210.

[Discharge Pipe of Compressor]

The refrigerant pipe 50 further includes a discharge pipe 220 throughwhich the refrigerant compressed in the compressor 100 is discharged.The discharge pipe 220 may be understood as a pipe extending from adischarge portion of the compressor 100 to the first port 111 of theflow control valve 110.

The discharge pipe 220 may include the ductile stainless steel pipe.Also, since a high-pressure gas refrigerant flows through the dischargepipe 220, the discharge pipe 220 may have a relatively small outerdiameter of about 9.45 mm to about 9.60 mm. Likewise, when at least twopipes are connected to each other, and then one pipe is expanded, thedischarge pipe 220 may have an outer diameter corresponding to an outerdiameter of the expanded pipe.

For example, the discharge pipe 220 may have an outer diameter of about9.52 mm. Also, the discharge pipe 220 may have an inner diameter ofabout 9.04 or less in consideration of a thickness of the discharge pipe220.

Since the high-pressure gas refrigerant flows through the discharge pipe220, and thus the discharge pipe 220 largely moves by vibrationoccurring in the compressor 100, it is necessary to maintain thestrength of the discharge pipe 220 to preset strength or more. When thedischarge pipe 220 is provided as the new material pipe, the dischargepipe 220 may be maintained at high strength to prevent the refrigerantfrom leaking by the damage of the discharge pipe 220.

A relatively low-pressure refrigerant flows through the suction pipe210, but the pipe is disposed adjacent to the compressor 100, themovement due to the vibration of the compressor 100 may be largelylarge. Thus, since the strength of the suction pipe 210 is required tobe maintained to the preset strength or more, the suction pipe 210 maybe provided as the new material pipe.

Hereinafter, constituents defining the characteristics of the ductilestainless steel according to an embodiment will be described. It isnoted that the constitutional ratios of the constituents described beloware weight percent (wt. %).

FIG. 3 is a microstructure photograph of a stainless steel having anaustenite matrix structure of about 99% and a delta ferrite structure ofabout 1% or less, and FIG. 4 is a microstructure photograph of astainless steel having only the austenite matrix structure.

1. Composition of Stainless Steel

(1) Carbon (C): 0.3% or less

The stainless steel according to an embodiment includes carbon (C) andchromium (Cr). Carbon and chromium react with each other to precipitateinto chromium carbide. Here, the chromium is depleted around a grainboundary or the chromium carbide to cause corrosion. Thus, the carbonmay be maintained at a small content.

Carbon is an element that is bonded to other elements to act to increasecreep strength. Thus, in the content of carbon exceeds about 0.93%, theductility may be deteriorated. Thus, the content of the carbon is set toabout 0.03% or less.

(2) Silicon (Si): more than 0% and less than 1.7%

An austenite structure has yield strength less that that of a ferritestructure or martensite structure. Thus, a matrix structure of thestainless steel may be made of austenite so that the ductile stainlesssteel according to an embodiment has a bending property (degree offreedom of bending) equal or similar to that of the copper.

However, silicon is an element forming ferrite, the more a content ofsilicon increases, the more a ratio of the ferrite in the matrixstructure increases to improve stability of the ferrite. It ispreferable that the silicon is maintained to be the content of siliconas low as possible, but it is impossible to completely blockintroduction of silicon into impurities during the manufacturingprocess.

When a content of silicon exceeds about 1.7%, the stainless steel hashardly ductility at a level of the copper material, and also, it isdifficult to secure sufficient workability. Thus, a content of siliconcontained in the stainless steel according to an embodiment is set toabout 1.7% or less.

(3) Manganese: 1.5% to 3.5%

Manganese acts to suppress phase transformation of the matrix structureof the stainless steel into a martensite type material and expand andstabilize an austenite region. If a content of manganese is less thanabout 1.5%, the phase transformation suppressing effect by manganesedoes not sufficiently occur. Thus, to sufficiently obtain the phasetransformation suppressing effect by manganese, a lower limit of acontent of manganese is set to about 1.5% or less.

However, as the content of manganese increases, the yield strength ofthe stainless steel increases to deteriorate the ductility of thestainless steel. Thus, an upper limit of the content of manganese is setto about 3.5%.

(4) Chromium (Cr): 15% to 18%

Chromium is an element that improves corrosion initiation resistance ofthe stainless steel. The corrosion initiation refers to first occurrenceof the corrosion in a state in which the corrosion does not exist in abase material, and the corrosion initiation resistance refers to aproperty of suppressing the first occurrence of the corrosion in thebase material. This may be interpreted to have the same means ascorrosion resistance.

Since the stainless steel does not have the corrosion initiationresistance (corrosion resistance) when a content of chromium is lessthan about 15.0%, a lower limit of the content of chromium is set toabout 15.0%.

On the other hand, if the content of chromium is too large, the ferritestructure is formed at room temperature to reduce the ductility.Particularly, the stability of the austenite is lost at a hightemperature to reduce the strength. Thus, an upper limit of the contentof the chromium is set to about 18.0% or less.

(5) Nickel (Ni): 7.0% to 9.0%

Nickel has a property of improving corrosion growth resistance of thestainless steel and stabilizing the austenite structure.

Corrosion growth refers to growth of corrosion that already occurs inthe base material while spreading over a wide range, and the corrosiongrowth resistance refers to a property of suppressing the growth of thecorrosion.

Since the stainless steel does not have the corrosion growth resistancewhen a content of nickel is less than about 7.0%, a lower limit of thecontent of nickel is set to about 7.0%.

Also, when the content of nickel is excessive, the stainless steelincreases in strength and hardness, and thus it is difficult to securesufficient workability of the stainless steel. In addition, the costincrease, and thus it is not desirable economically. Thus, an upperlimit of the content of the nickel is set to about 9.0% or less.

(6) Copper (Cu): 1.0% to 4.0%

Copper acts to inhibit phase transformation of the matrix structure ofthe stainless steel into a martensite structure and improve theductility of the stainless steel. If a content of copper is less thanabout 1.0%, the phase transformation suppressing effect by copper doesnot sufficiently occur. Thus, to sufficiently obtain the phasetransformation suppressing effect by copper, a lower limit of a contentof copper is set to about 1.0% or less.

Particularly, the content of copper has to set to about 1.0% or more sothat the stainless steel has a bending property equal or similar to thatof the copper.

Although the more the content of copper increases, the more the phasetransformation suppressing effect of the matrix structure increases, theincrease gradually decreases. Also, if the content of copper isexcessive to exceed about 4% to about 4.5%, since the effect issaturated, and the occurrence of martensite is promoted, it is notpreferable. Also, since copper is an expensive element, it affectseconomical efficiency. Thus, an upper limit of the content of copper isset to about 4.0% so that the effect of suppressing the phasetransformation of copper is maintained to the saturation level, and theeconomical efficiency is secured.

(7) Molybdenum (Mo): 0.03% or less

(8) Phosphorus (P): 0.04% or less

(9) Sulfur (S): 0.04% or less

(10) Nitrogen (N): 0.03% or less

Since molybdenum, phosphorus, sulfur, and nitrogen are elementsoriginally contained in the steel-finished product and cure thestainless steel, it is desirable to maintain the contents as low aspossible.

2. Matrix Structure of Stainless Steel

When the stainless steel is classified in view of a metal structure (ormatrix structure), the stainless steel is classified into austenite typestainless steel containing chromium (18%) and nickel (8%) as maincomponents and ferrite type stainless steel containing chromium (18%) asa main component, and martensite type stainless steel containingchromium (8%) as a main component.

Also, since the austenite type stainless steel is excellent in corrosionresistance against salt and acid and has high ductility, the ductilestainless steel according to an embodiment is preferably the austenitetype stainless steel.

Also, the austenite structure has yield strength and hardness less thatthose of the ferrite structure or the martensite structure. Furthermore,when a crystal size is grown under the same condition, an average grainsize of the austenite is the largest and thus is advantageous forimproving the ductility.

To improve the ductility of the stainless steel, the matrix structure ofthe stainless steel may be formed as only the austenite structure.However, since it is very difficult to control the matrix structure ofthe stainless steel with only the austenite, it is inevitable to includeother structure.

In detail, the other matrix structure that affects the ductility of theaustenite type stainless steels is delta ferrite (δ-ferrite) whichoccurs during the heat treatment process. That is, the more a content ofthe delta ferrite, the more the hardness of the stainless steelincreases, but the ductility of the stainless steel decreases.

The stainless steel may have an austenite matrix structure of about 90%or more, preferably about 99% or more and a delta ferrite matrixstructure of about 1% or more on the base of a grain area. Thus, one ofmethods for improving the ductility of the stainless steels is to reducean amount of delta ferrite contRained in the austenite type stainlesssteel.

Even when the ductile stainless steel according to an embodiment has adelta ferrite matrix structure of about 1% or less, the fact that thedelta ferrite is locally distributed in a specific crystal grain ratherthan being uniformly distributed throughout the crystal grain isadvantageous in improvement of the ductility.

[Microstructure of Ductile Stainless Steel]

FIG. 3 is a microstructure photograph of a stainless steel having anaustenite matrix structure of about 99% and a delta ferrite structure ofabout 1% or less, and FIG. 4 is a microstructure photograph of astainless steel having only the austenite matrix structure. Thestainless steel having the structure of FIG. 3 is a microstructure ofthe ductile stainless steel according to an embodiment.

The stainless steel of FIG. 3 and the stainless steel of FIG. 4 haveaverage grain sizes corresponding to grain size Nos. 5.0 to 7.0. Theaverage gain size will be descried below.

Table 1 below is a graph of results obtained by comparing mechanicalproperties of the stainless steel (a material 1) of FIG. 3 and thestainless steel (a material 2) of FIG. 3.

TABLE 1 Mechanical Property Yield Tensile Strength Strength HardnessElongation Kind [MPa] [MPa] [Hv] [%] Material 1 Stainless Steel 180 500120 52 (Austenite + Delta Ferrite) Material 2 Stainless Steel 160 480110 60 (Austenite)

Referring to Table 1, it is seen that the material 2 has a physicalproperty less than that of the material 1 in strength and hardness.Also, it is seen that the material 2 has an elongation greater than thatof the material 1. Therefore, to lower the strength and the hardness ofthe stainless steel, it is ideal that the stainless steel has only theaustenite matrix structure. However, since it is difficult to completelyremove the delta ferrite matrix structure, it is desirable to minimize aratio of the delta ferrite matrix structure.

Also, as described above, when the delta ferrite structures are denselydistributed in a specific grain rather than uniformly distributed, theeffect is more effective for the ductility the stainless steel.

In FIG. 3, a large grain 101 represents an austenite matrix structure,and a small grain 102 in the form of a black spot represents a deltaferrite matrix structure.

3. Average Grain Diameter of Stainless Steel

An average grain diameter of the stainless steel may be determinedaccording to composition and/or heat treatment conditions. The averagegrain diameter of the stainless steel affects the strength and thehardness of the stainless steel. For example, the more the average graindiameter decreases, the more the stainless steel increase in strengthand hardness, and the more the average grain diameter increases, themore the stainless steel decrease in strength and hardness.

The ductile stainless steel according to an embodiment has properties oflow strength and hardness when compared to the stainless steel accordingto the related art in addition to good bending property by controllingthe content of copper and the grain area of delta ferrite, and also, theductile stainless steel has strength and hardness greater than those ofcopper.

For this, the average grain diameter of the stainless steel is limitedto about 30 μm to about 60 μm. An average grain diameter of a generalaustenite structure is less than about 30 μm. Thus, the average graindiameter has to increase to about 30 μm through the manufacturingprocess and the heat treatment.

According to the criteria of American Society for Testing and Materials(ASTM), the average grain diameter of about 30 μm to about 60 μmcorresponds to grain size Nos. 5.0 to 7.0. On the other hand, an averagegrain diameter less than about 30 μm corresponds to ASTM grain size No.7.5 or more.

If the average grain diameter of the stainless steel is less than about30 μm, or the grain size number is greater than 7.0, it does not havethe properties of low strength and low hardness required in thisembodiment. Particularly, the average grain diameter (or the grain sizenumber) of the stainless steel is a key factor in determining theproperties of the low strength and the low hardness of the stainlesssteel.

Referring to Table 2 below, since the copper pipe according to therelated art has physical properties of the low strength and the lowhardness, the copper pipe is commercialized as the refrigerant pipeconstituting the refrigerant circulation cycle, but there is alimitation of reliability due to the corrosion and pressure resistanceagainst a new refrigerant.

Also, since the stainless steels of Comparative Examples 2 to 5 haveexcessively large strength and hardness in comparison to the copperpipes, there is a limitation that the workability is poor even if thelimitation of the corrosion and the pressure resistance of copper aresolved.

On the other hand, the stainless steel according to an embodiment hasstrength and hardness greater than those the copper pipes according tothe related art and has strength and hardness less than those of thestainless steels of Comparative Examples 2 to 5. Therefore, since thecorrosion resistance and the pressure resistance of the copper pipe aresolved, it is suitable to be used as a high-pressure new refrigerantpipe such as R32.

In addition, since it has an elongation greater than that of the copperpipe, the limitation of workability of the stainless steel according tothe related art may also be solved.

TABLE 2 Mechanical Property Tensile Yield Strength Strength HardnessElongation Kind [MPa] [MPa] [Hv] [%] Comparative Copper Pipe 100 270 10045 or more Example 1 (C1220T) Comparative Stainless Steel about 200about 500 about 130 50 or more Examples 2-5 (Grain Size No. 7.5 or more)Embodiment Stainless Steel about 160 about 480 about 120 60 or more(Grain Size No. 5.0~7.0)

In summary, the ductile stainless steel defined in an embodiment mayrepresent stainless steel which has about 99% of austenite and about 1%or less of delta ferrite and in which the above-described components arecontained at a preset ratio.

FIG. 5 is a view illustrating an outer diameter and an inner diameter ofthe refrigerant pipe according to an embodiment.

Referring to FIGS. 2 and 5, when the compressor 100 according to anembodiment is driven, the refrigerant suctioned into the compressor 100involves a temperature change after the compression. Due to the changein temperature, a change in stress at the suction pipe 210 and thedischarge pipe 220 may be more severe than other pipes.

As illustrated in FIG. 4, this embodiment is characterized in that thesuction pipe 210 and the discharge pipe 220, which exhibit the mostsevere pressure and vibration when the refrigerant changes in phase, areformed as the ductile stainless steel pipe subjected to a ductilenessprocess to increase allowable stress. However, the present disclosure isnot limited to only the suction pipe and the discharge pipe, and any oneor more pipes connecting the outdoor unit to the indoor unit may beprovided as the ductile stainless steel pipe according to the variationof the stress.

The air-conditioning capacity of the air conditioner 10 according to anembodiment may be selected in the range of about 2.5 kW to about 15 kW.An outer diameter of the ductile stainless steel pipe may be determinedbased on the selected air-conditioning capacity of the air conditioner10.

Also, the refrigerant used in the air conditioner 10 according to anembodiment may include the R32, the R134a, or the R401a as describedabove. Particularly, a thickness of the ductile stainless steel pipe maybe differently determined according to kinds of refrigerants.

[Method for Setting Thickness of Ductile Stainless Steel Pipe]

A thickness of the ductile stainless steel pipe may be determinedaccording to the following Mathematical Equation. The MathematicalEquation below is calculated based on ASME B31.1, which provides codesfor standards and guidelines for a pipe, and KGS Code, which categorizestechnical items such as facilities, technologies, and inspectionsspecified by gas related laws and regulations.

$\begin{matrix}{t_{m} = {\frac{P \times D_{0}}{{2\; S} + {0.8\; P}} + T_{extra}}} & \left\lbrack {{Mathematical}\mspace{14mu} {Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Here, tm represents a minimum thickness of the stainless steel pipe, Prepresents a design pressure (Mpa), D0 represents an outer diameter (mm)of the stainless steel pipe, S represents allowable stress (M/mm2), andTextra represents a clearance thickness according to corrosion, threadworking, and the like. The Textra is determined to be about 0.2 when amaterial of the pipe is made of copper, aluminum, or stainless steel.

[Definition of Pipe Diameter]

As illustrated in FIG. 5, an outer diameter of the ductile stainlesssteel pipe used for the suction pipe 210 or the discharge pipe 220 maybe defined as a, and an inner diameter may be defined as b. Referring toMathematical Equation 1, it is seen that the minimum thickness of thepipe is proportional to the outer diameter of the pipe and inverselyproportional to the allowable stress.

[Allowable Stress S]

The allowable stress represents a value obtained by dividing referencestrength by a safety factor, i.e., a maximum value of stress(deformation force) that is allowed to exert weight, which is consideredto be tolerable without deformation or breakage of the pipe whenexternal force is applied to the pipe.

In this embodiment, the allowable stress standard of the ductilestainless steel pipe is derived to satisfy the code written in ASME SEC.VIII Div. 1, and the allowable stress S may be set to a relatively smallvalue of a value obtained by dividing the tensile strength of the pipeby 3.5 or a value obtained by dividing the yield strength of the pipe by1.5. The allowable stress may be a value that varies depending on thematerial of the pipe and be determined to about 93.3 Mpa on the basis ofthe SME SEC. VIII Div. 1.

When the same stress is applied to the pipe, the stainless steel mayhave a stress margin greater than that of copper, and thus a degree ofdesign freedom of the pipe may increase. As a result, to reduce thestress transmitted to the pipe, it is possible to escape the restrictionthat the pipe has to have a long length. For example, to reducevibration transmitted from the compressor 100, it is unnecessary to bendthe pipe several times in the form of a loop within a limitedinstallation space.

[Outer Diameter of Ductile Stainless Steel Pipe]

Air-conditioning capacity of the air conditioner 10, i.e., coolingcapacity or heating capacity may be determined based on compressibilityof the compressor 100. Also, an outer diameter of the ductile stainlesssteel pipe may be determined according to the cooling capacity of thecompressor. That is, the capacity of the compressor may be a criterionfor determining the outer diameter of the ductile stainless steel pipe.

For example, in the air conditioner 10 having air-conditioning capacityof about 2.5 kW to about 3.5 kW, when the suction pipe 210 and thedischarge pipe 220 are provided as the ductile stainless steel pipes,the suction pipe 210 may have an outer diameter of about 12.6 mm toabout 12.8 mm, and the discharge pipe 220 may have an outer diameter ofabout 9.45 mm to about 9.60 mm.

For another example, in the air conditioner 10 having air-conditioningcapacity of about 3.5 kW to about 5 kW, when the suction pipe 210 andthe discharge pipe 220 are provided as the ductile stainless steelpipes, the suction pipe 210 may have an outer diameter of about 12.60 mmto about 12.80 mm, and the discharge pipe 220 may have an outer diameterabout 9.45 mm to about 9.60 mm.

For another example, in the air conditioner 10 having air-conditioningcapacity of about 5 kW to about 7 kW, when the suction pipe 210 and thedischarge pipe 220 are provided as the ductile stainless steel pipes,the suction pipe 210 may have an outer diameter of about 15.80 mm toabout 16.05 mm, and the discharge pipe 220 may have an outer diameterabout 9.45 mm to about 9.8 mm.

For another example, in the air conditioner 10 having air-conditioningcapacity of about 7 kW to about 8 kW, when the suction pipe 210 and thedischarge pipe 220 are provided as the ductile stainless steel pipes,the suction pipe 210 may have an outer diameter of about 15.80 mm toabout 15.95 mm, and the discharge pipe 220 may have an outer diameterabout 9.45 mm to about 9.60 mm.

For another example, in the air conditioner 10 having air-conditioningcapacity of about 12 kW to about 17 kW, when the suction pipe 210 andthe discharge pipe 220 are provided as the ductile stainless steelpipes, the suction pipe 210 may have an outer diameter of about 17.40 mmto about 19.10 mm, and the discharge pipe 220 may have an outer diameterabout 12.60 mm to about 12.80 mm.

The air conditioner 10 according to this embodiment may haveair-conditioning capacity of about 7 kW to about 8 kW.

[Design Pressure P According to Kind of Refrigerant]

A design pressure may be a pressure of the refrigerant and correspond toa condensation pressure of the refrigerant cycle. For example, thecondensation pressure may be determined based on a temperature value(hereinafter, referred to as a condensation temperature) of therefrigerant condensed in the outdoor heat exchanger 120 or the indoorheat exchanger. Also, the design pressure may represent a saturatedvapor pressure of the refrigerant at the condensation temperature. Ingeneral, the air conditioner may have a condensation temperature ofabout 65° C.

The saturated vapor pressure according to kinds of refrigerants is shownin Table 3.

TABLE 3 Refrigerant Temperature R134a R410a R32 (° C.) (Mpa) (Mpa) (Mpa)−20 0.03 0.30 0.30 0 0.19 0.70 0.71 20 1.47 1.35 1.37 40 0.91 2.32 1.4760 1.58 3.73 3.85 65 1.79 4.15 4.30

Referring to Table 3, when the R410A is used as the refrigerant, asaturated vapor pressure at about 65° C. is 4.15, and thus the designpressure P may be determined to about 4.15 (MPa).

[Calculation of Minimum Thickness of Ductile Stainless Steel PipeThrough which R410a Flows]

As described above, the allowable stress S is given by ASME SEC. VIIIDiv. 1, and the design pressure P is determined to about 4.15 MPa whenthe refrigerant is R410a, and the refrigerant temperature is about 65°C. A minimum thickness of the pipe, which is calculated according to theouter diameter of the pipe by applying the determined allowable stress Sand the design pressure P to Mathematical Equation 1 may be confirmed bythe following Table 4.

TABLE 4 Minimum Thickness (mm) Outer Embodiment Calculated MinimumDiameter to which margin Comparative Thickness (R410a) of is applied(ductile Example ASME Standard stainless steel pipe) (copper B31.1 JIS B8607 Pipe R410a pipe) (t_(m)) (t_(m) − t_(exrta)) φ4.00 0.40 0.30 0.10φ4.76 0.40 0.32 0.12 φ5.00 0.40 0.33 0.13 φ6.35 0.40 0.622 0.36 0.16φ7.00 0.40 0.38 0.18 φ7.94 0.50 0.622 0.40 0.20 φ9.52 0.50 0.622 0.440.24 φ12.70 0.60 0.622 0.53 0.33 φ15.88 0.70 0.800 0.61 0.41 φ19.05 0.800.800 0.69 0.49 φ22.20 1.00 1.041 0.77 0.57 φ25.40 1.00 1.168 0.85 0.65φ28.00 1.00 1.168 0.92 0.72 φ31.80 1.20 1.283 1.01 0.81 φ34.90 1.201.283 1.09 0.89 φ38.10 1.20 1.410 1.18 0.98 φ41.28 1.20 1.410 1.26 1.06φ50.80 1.50 1.50 1.30 φ54.00 1.50 1.623 1.58 1.38

Referring to Table 4, a minimum thickness of the ductile stainless steelpipe derived based on ASME B31.1 and a minimum thickness of the ductilestainless steel pipe derived based on JIS B 8607 may be confirmed. Here,in an embodiment, the ductile stainless steel pipe was used, and inComparative example, the existing copper pipe was used.

JIS B 8607 is a reference code for a pipe used in Japan. In case of JISB 8607, a minimum thickness is derived to be less than that in case ofASME B31.1 because the textra value that is the clearance thickness dueto corrosion and the thread working is not considered, unlike ASMEB31.1. The textra value may be set to about 0.2 mm in case of copper, acopper alloy, aluminum, an aluminum alloy, and stainless steel.

Although the minimum thickness of the ductile stainless steel pipeaccording to an embodiment is derived based on ASME B31.1, the minimumthickness may be applicable with a predetermined margin determinedbetween about 0.1 mm to about 0.2 mm in consideration of the pressurewhen the R401 a as the refrigerant. That is, an embodiment is understoodthat the minimum thickness is suggested with a margin as one example. Ifthe minimum thickness is greater than the calculated minimum thickness,the margin may vary based on the safety factor.

Particularly, in case of the same outer diameter (φ7.94) in Table 4, itis confirmed that the applicable pipe thickness according to anembodiment is about 0.50 mm, and the applicable pipe thickness accordingto Comparative Example is about 0.622 mm That is, when a pipe designedto have the same outer diameter is provided as the ductile stainlesssteel pipe described in the embodiment, it means that the thickness ofthe pipe may be further reduced, and also this means that an innerdiameter of the pipe may further increase.

The suction pipe 210 has an outer diameter of about 15.80 mm to about15.95 mm Referring to Table 4, the standard pipe of the suction pipe 210has an outer diameter of about 15.88 mm, and the suction pipe 210 has aminimum thickness of about 0.61 mm in case of ASME B31.1, about 0.41 mmin case of JIS B 8607, and about 0.70 mm in case of an embodiment towhich a margin is applied.

Thus, a limit thickness value, which is applicable to the suction pipe210, of the above criteria is about 0.41 mm on the basis of JIS B 8607.As a result, the suction pipe 210 may have an inner diameter of about15.06 mm (=15.88−2×0.41) or less.

Also, the discharge pipe 220 has an outer diameter of about 9.45 mm toabout 9.60 mm Referring to Table 4, the standard pipe of the dischargepipe 220 has an outer diameter of about 9.52 mm, and the discharge pipe220 has a minimum thickness of about 0.44 mm in case of ASME B31.1,about 0.24 mm in case of JIS B 8607, and about 0.50 mm in case of anembodiment to which a margin is applied.

Thus, a limit thickness value, which is applicable to the discharge pipe210, of the above criteria is about 0.24 mm on the basis of JIS B 8607.As a result, the discharge pipe 220 may have an inner diameter of about9.04 mm (=9.52−2×0.24) or less.

[Calculation of Minimum Thickness of Ductile Stainless Steel PipeThrough which R32 Flows]

As described above, the allowable stress S is given by ASME SEC. VIIIDiv. 1, and the design pressure P is determined to about 4.30 MPa whenthe refrigerant is R32, and the refrigerant temperature is about 65° C.A minimum thickness of the pipe, which is calculated according to theouter diameter of the pipe by applying the determined allowable stress Sand the design pressure P to Mathematical Equation 1 may be confirmed bythe following Table 5.

TABLE 5 Minimum Thickness (mm) Calculated Minimum Outer DiameterEmbodiment Comparative Thickness (R32) of (ductile stainless ExampleASME Standard steel pipe) (copper B31.1 JIS B 8607 Pipe R32 pipe)(t_(m)) (t_(m) − t_(exrta)) φ4.00 0.40 0.30 0.10 φ4.76 0.40 0.32 0.12φ5.00 0.40 0.34 0.14 φ6.35 0.40 0.622 0.38 0.18 φ7.00 0.50 0.41 0.21φ7.94 0.50 0.622 0.44 0.24 φ9.52 0.50 0.622 0.48 0.28 φ12.70 0.60 0.6220.56 0.36 φ15.88 0.70 0.800 0.65 0.45 φ19.05 0.80 0.800 0.73 0.53 φ22.201.00 1.041 0.81 0.61 φ25.40 1.00 1.168 0.89 0.69 φ28.00 1.00 1.168 0.960.76 φ31.80 1.10 1.283 1.06 0.86 φ34.90 1.20 1.283 1.14 0.94 φ38.10 1.301.410 1.22 1.02 φ41.28 1.40 1.410 1.30 1.10 φ50.80 1.70 1.54 1.34 φ54.001.70 1.623 1.61 1.41

Referring to [Table 5], a minimum thickness of the ductile stainlesssteel pipe derived based on ASME B31.1 and a minimum thickness of theductile stainless steel pipe derived based on JIS B 8607 may beconfirmed. Here, in an embodiment, the ductile stainless steel pipe wasused, and in Comparative example, the existing copper pipe was used.

JIS B 8607 is a reference code for a pipe used in Japan. In case of JISB 8607, a minimum thickness is derived to be less than that in case ofASME B31.1 because the textra value that is the clearance thickness dueto corrosion and the thread working is not considered, unlike ASMEB31.1. The textra value may be set to about 0.2 mm in case of copper, acopper alloy, aluminum, an aluminum alloy, and stainless steel.

Although the minimum thickness of the ductile stainless steel pipeaccording to an embodiment is derived based on ASME B31.1, the minimumthickness may be applicable with a predetermined margin determinedbetween about 0.1 mm to about 0.2 mm in consideration of the pressurewhen the R32 as the refrigerant. That is, an embodiment is understoodthat the minimum thickness is suggested with a margin as one example. Ifthe minimum thickness is greater than the calculated minimum thickness,the margin may vary based on the safety factor.

Particularly, in case of the same outer diameter (φ7.94) in Table 5, itis confirmed that the applicable pipe thickness according to anembodiment is about 0.50 mm, and the applicable pipe thickness accordingto Comparative Example is about 0.622 mm That is, when a pipe designedto have the same outer diameter is provided as the ductile stainlesssteel pipe described in the embodiment, it means that the thickness ofthe pipe may be further reduced, and also this means that an innerdiameter of the pipe may further increase.

In this embodiment, the suction pipe 210 has an outer diameter of about19.00 mm to about 19.20 mm Referring to Table 5, the standard pipe ofthe suction pipe 210 has an outer diameter of about 19.05 mm, and thesuction pipe 210 has a minimum thickness of about 0.73 mm in case ofASME B31.1, about 0.53 mm in case of JIS B 8607, and about 0.80 mm incase of an embodiment to which a margin is applied.

Thus, a limit thickness value, which is applicable to the suction pipe210, of the above criteria is about 0.53 mm on the basis of JIS B 8607.As a result, the suction pipe 210 may have an inner diameter of about17.99 mm (9.05−0.53×2) or less.

Also, the discharge pipe 220 has an outer diameter of about 12.60 mm toabout 12.80 mm Referring to Table 5, the standard pipe of the dischargepipe 220 has an outer diameter of about 12.70 mm, and the discharge pipe220 has a minimum thickness of about 0.56 mm in case of ASME B31.1,about 0.36 mm in case of JIS B 31.1, and about 0.60 mm in case of anembodiment to which a margin is applied.

Thus, a limit thickness value, which is applicable to the discharge pipe220, of the above criteria is about 0.36 mm on the basis of JIS B 8607.As a result, the discharge pipe 220 may have an inner diameter of about11.98 mm (12.70−0.36×2) or less.

[Calculation of Minimum Thickness of Ductile Stainless Steel PipeThrough which R134a Flows]

As described above, the allowable stress S is given by ASME SEC. VIIIDiv. 1, and the design pressure P is determined to about 1.79 MPa whenthe refrigerant is R134a, and the refrigerant temperature is about 65°C. A minimum thickness of the pipe, which is calculated according to theouter diameter of the pipe by applying the determined allowable stress Sand the design pressure P to Mathematical Equation 1 may be confirmed bythe following Table 6.

TABLE 6 Minimum Thickness (mm) Embodiment Outer (ductile CalculatedMinimum Diameter of stainless Comparative Thickness (R134a) Standardsteel pipe) Example ASME B31.1 JIS B 8607 Pipe R134a (copper pipe)(t_(m)) (t_(m) − t_(exrta)) φ4.00 0.40 0.25 0.05 φ4.76 0.40 0.26 0.06φ5.00 0.40 0.27 0.07 φ6.35 0.40 0.622 0.30 0.10 φ7.00 0.40 0.32 0.12φ7.94 0.40 0.622 0.34 0.14 φ9.52 0.40 0.622 0.36 0.16 φ12.70 0.50 0.6220.40 0.20 φ15.88 0.50 0.800 0.45 0.25 φ19.05 0.50 0.800 0.49 0.29 φ22.200.60 1.041 0.53 0.33 φ25.40 0.60 1.168 0.57 0.37 φ28.00 0.70 1.168 0.610.41 φ31.80 0.70 1.283 0.66 0.46 φ34.90 0.80 1.283 0.70 0.50 φ38.10 0.801.410 0.74 0.54 φ41.28 1.00 1.410 0.78 0.58 φ50.80 1.00 0.89 0.69 φ54.001.20 1.623 0.93 0.73

Referring to Table 6, a minimum thickness of the ductile stainless steelpipe derived based on ASME B31.1 and a minimum thickness of the ductilestainless steel pipe derived based on JIS B 8607 may be confirmed. Here,in an embodiment, the ductile stainless steel pipe was used, and inComparative example, the existing copper pipe was used.

JIS B 8607 is a reference code for a pipe used in Japan. In case of JISB 8607, a minimum thickness is derived to be less than that in case ofASME B31.1 because the textra value that is the clearance thickness dueto corrosion and the thread working is not considered, unlike ASMEB31.1. The textra value may be set to about 0.2 mm in case of copper, acopper alloy, aluminum, an aluminum alloy, and stainless steel.

Although the minimum thickness of the ductile stainless steel pipeaccording to an embodiment is derived based on ASME B31.1, the minimumthickness may be applicable with a predetermined margin determinedbetween about 0.1 mm to about 0.2 mm in consideration of the pressurewhen the R134a as the refrigerant. That is, an embodiment is understoodthat the minimum thickness is suggested with a margin as one example. Ifthe minimum thickness is greater than the calculated minimum thickness,the margin may vary based on the safety factor.

Particularly, in case of the same outer diameter (φ7.94) in Table 6, itis confirmed that the applicable pipe thickness according to anembodiment is about 0.40 mm, and the applicable pipe thickness accordingto Comparative Example is about 0.622 mm That is, when a pipe designedto have the same outer diameter is provided as the ductile stainlesssteel pipe described in the embodiment, it means that the thickness ofthe pipe may be further reduced, and also this means that an innerdiameter of the pipe may further increase.

In this embodiment, the suction pipe 210 has an outer diameter of about19.00 mm to about 19.20 mm Referring to Table 6, the standard pipe ofthe suction pipe 210 has an outer diameter of about 19.05 mm, and thesuction pipe 210 has a minimum thickness of about 0.49 mm in case ofASME B31.1, about 0.29 mm in case of JIS B 8607, and about 0.50 mm incase of an embodiment to which a margin is applied.

Thus, a limit thickness value, which is applicable to the suction pipe210, of the above criteria is about 0.29 mm on the basis of JIS B 8607.As a result, the suction pipe 210 may have an inner diameter of about18.47 mm (9.05−0.29×2) or less.

Also, the discharge pipe 220 has an outer diameter of about 12.60 mm toabout 12.80 mm Referring to Table 6, the standard pipe of the dischargepipe 220 has an outer diameter of about 12.70 mm, and the discharge pipe220 has a minimum thickness of about 0.40 mm in case of ASME B31.1,about 0.20 mm in case of JIS B 31.1, and about 0.50 mm in case of anembodiment to which a margin is applied.

Thus, a limit thickness value, which is applicable to the discharge pipe220, of the above criteria is about 0.20 mm on the basis of JIS B 8607.As a result, the discharge pipe 220 may have an inner diameter of about12.30 mm (12.70−0.20×2) or less.

In summary, the outer diameter of the pipe used in the compressor 100according to an embodiment may be determined by the refrigerationcapacity of the compressor or the air-conditioning capacity of the airconditioner 10, and the design pressure may be determined according tothe used refrigerant.

In case where the suction pipe and the discharge pipe are provided asthe ductile stainless steel pipes described in the embodiment, since theallowable stress of the stainless steel is greater than that of copper,it is seen that the thickness of the pipe is reduced by applying therelatively large allowable stress to Mathematical Equation 1. That is,the ductile stainless steel pipe having relatively high strength orhardness may be used to increase the allowable stress, and thus, athickness at the same outer pipe diameter may be reduced.

Thus, even though the ductile stainless steel pipe according to anembodiment is designed to have the same outer diameter as that of thecopper pipe according to the related art, the inner diameter may bedesigned to be larger to reduce flow resistance of the refrigerant,thereby improving the circulation efficiency of the refrigerant.

FIG. 6 is a flowchart illustrating a method for manufacturing theductile stainless steel pipe according to an embodiment, FIG. 7 is aschematic view of a cold rolling process S1 of FIG. 6, FIG. 8 is aschematic view of a slitting process S2 of FIG. 6, FIG. 9 is a schematicview of a forming process S3 of FIG. 6, FIGS. 10 to 13 arecross-sectional views illustrating a process of manufacturing a ductilestainless steel pipe according to the manufacturing method of FIG. 6,and FIG. 14 is a schematic view of a bright annealing process of FIG. 6.

As described above, since the stainless steel according to the relatedart has strength and hardness grater than those of copper and thus has alimitation of workability. Particularly, there is a limitation that thestainless steel is limited in bending.

[Required Property of Ductile Stainless Steel Pipe]

To solve these limitations, since the ductile stainless steel pipeaccording to an embodiment has a composition containing copper, a matrixstructure made of austenite, and an average grain size of about 30 μm toabout 60 μm, the ductile stainless steel pipe may have strength andhardness less than those of the stainless steel pipe according to therelated art.

Particularly, the austenite has low resistive abdominal strength and lowhardness properties when compared to ferrite or martensite. Thus, tomanufacture the ductile stainless steel pipe having the properties ofthe low strength and the low hardness required in this embodiment, it isrequired to have an austenite matrix structure of about 99% or more anda delta ferrite matrix structure of about 1% or less on the base of thegrain area of the ductile stainless steel pipe.

For this, the ductile stainless steel pipe may have austenite matrixstructure of about 99% or more and the delta ferrite matrix structure ofabout 1% or less on the base of the grain area of the ductile stainlesssteel pipe by applying the composition ratio and performing anadditional heat treatment.

[Heat Treatment Process of Ductile Stainless Steel Pipe]

A heat treatment process of the ductile stainless steel pipe will bedescribed in detail.

Unlike that the pipe made of copper is manufactured by a single processsuch as drawing, it is difficult to manufacture the pipe made of theductile stainless steel through a single process because of havingstrength and hardness greater than those of copper.

The heat treatment process of the ductile stainless steel pipe accordingto an embodiment may include a cold rolling process S1, a slittingprocess S2, a forming process S3, a welding process S4, a cuttingprocess S5, a drawing process S6, and a bright annealing process S7.

[First Process: Cold Rolling Process S1]

The cold rolling process S1 may be understood as a process for rollingthe ductile stainless steel provided in the casting process by passingthrough two rotating rolls at a temperature below a recrystallizationtemperature. That is, in the cold-rolled ductile stainless steel,unevenness or wrinkles on a surface of a thin film may be corrected, andsurface gloss may be given on the surface.

As illustrated in FIG. 7, the ductile stainless steel is provided in theform of a sheet 310, and the sheet 310 is provided to be wound in a coilshape by an uncoiler.

The sheet 310 may receive continuous force by passing between the tworotating rolling rolls 320 disposed in a vertical direction, and thusthe sheet 310 may be widened in surface area and thinned in thickness.In this embodiment, the ductile stainless steel is provided in the formof a sheet having a thickness of about 1.6 mm to about 3 mm in thecasting process, and the sheet may be cold-worked to a sheet having athickness of about 1 mm or less through the cold rolling process S1.

[Second Process: Slitting Process S2]

The slitting process S2 may be understood as a process of cutting thecold-worked sheet 310 into a plurality of pieces having a desired widthby using a slitter. That is, the single sheet 310 may be cut and workedinto a plurality of pieces through the slitting process S2.

As illustrated in FIG. 8, the cold-worked sheet 310 may pass through theslitter 332 while the wound coil is unwound by the rotation of theuncoiler 331 in the state in which the sheet 310 is wound in a coilshape around an outer circumferential surface of the uncoiler 331.

For example, the slitter 332 may include a shaft that is disposed in thevertical direction of the sheet 310 and a rotational cutter 332 acoupled to the shaft. The rotational cutter 332 a may be provided inplurality, and the plurality of rotational cutters 332 may be spacedapart from each other in a width direction of the sheet 310. Spaceddistances between the plurality of rotational cutters 332 a may be thesame or different from each other in some cases.

Thus, when the sheet 310 passes through the slitter 332, the singlesheet 310 may be divided into a plurality of sheets 310 a, 310 b, 310 c,and 310 d by the plurality of rotational cutters 332 a. In this process,the sheet 310 may have a suitable diameter or width of the refrigerantpipe to be applied. Here, the sheet 310 may be pressed by a plurality ofsupport rollers 333 and 334 arranged in the vertical direction so as tobe precisely cut by the slitter 332.

When the slitting process S2 is completed, a bur may be formed on anouter surface of the sheet 310, and the bur needs to be removed. If thebur remains on the outer surface of the sheet 310, welding failure mayoccur in a process of welding the pipe worked in the form of the sheet310 to the other pipe, and the refrigerant may leak through a poorwelding portion. Accordingly, when the slitting process S2 is completed,a polishing process for removing the bur needs to be additionallyperformed.

[Third Process: Forming Process S3]

The forming process S3 may be understood as a process of molding theductile stainless steel in the form of a sheet 310 a by passing throughmulti-staged molding rolls 340 to manufacture the ductile stainlesssteel in the form of a pipe 310 a.

As illustrated in FIG. 9, in the state that the sheet 310 a is wound inthe form of the coil on the outer circumferential surface of theuncoiler, the coil wound by the rotation of the uncoiler is unwound toenter into the multi-staged forming rolls 340 that alternately disposedin the vertical or horizontal direction. The sheet 310 a entering intothe multi-staged molding rolls 340 may successively pass through themolding rolls 340 and thus be molded in the form of a pipe 310 e ofwhich both ends are adjacent to each other.

FIG. 10 illustrates a shape in which the ductile stainless steel havingthe sheet shape is rolled and then molded in the form of a pipe 10 e.That is, the ductile stainless steel having the form of the sheet 10 amay be molded into a pipe 310 e, of which both ends 311 a and 311 bapproach each other, through the forming process S3.

[Fourth Process: Welding Process S4]

The welding process S4 may be understood as a process of bonding boththe ends 311 a and 311 b of the pipe 310 e, which approach each other bybeing rolled by the forming process S3, to manufacture a welded pipe. Inthe welding process S4, the welded pipe may be realized by butt-weldingboth ends facing each other through a melting welding machine, forexample, a general electric resistance welding machine, an argon weldingmachine, or a high-frequency welding machine.

FIG. 11 illustrates a pipe manufactured by rolling and welding a sheetmade of ductile stainless steel. Particularly, both the ends 311 a and311 b of the pipe 310 e may be welded in a longitudinal direction of thepipe 310 e to bond both the ends 311 a and 311 b to each other.

Here, in the welding process, a weld zone 313 is formed in thelongitudinal direction of the pipe 310 e. As illustrated in FIG. 11,since beads 313 a and 313 b that slightly protrude from an innercircumferential surface 312 and an outer circumferential surface 311 ofthe pipe 310 e are formed at the weld zone 313, each of the innercircumferential surface 312 and the outer circumferential surface 311 ofthe pipe 310 e does not have a smooth surface.

Heat-affected zones 314 a and 314 b may be further formed on both sidesof the weld zone 313 by heat during the welding process. Theheat-affected zones 314 a and 314 b may also be formed in thelongitudinal direction of the pipe 310 e, like the weld zone 313.

[Fifth Process: Cutting Process S5]

The cutting process S5 may be understood as a process of partiallycutting the bead 313 a of the weld zone 313 so that the outercircumferential surface 311 of the pipe 310 e has the smooth surface.The cutting process S5 may be continuous with the welding process S4.

For example, the cutting process S5 may include a process of partiallycutting the bead 313 a using a bite while moving the pipe in thelongitudinal direction through press bead rolling.

FIG. 12 illustrates a ductile stainless steel pipe in which the cuttingprocess S5 is finished. That is, the bead 313 a formed on the outercircumferential surface 311 of the pipe 310 e may be removed through thecutting process S5. In some cases, the cutting process S5 may beperformed together with the welding process S4, whereas the cuttingprocess S5 may be omitted.

[Sixth Process: Drawing Process S6]

The drawing process S6 may be understood as a process of applyingexternal force to the bead 313 b of the weld zone 313 so that the outercircumferential surface 311 of the pipe 310 e has the smooth surface.

For example, the drawing process S6 may be performed by using a drawerincluding dies having a hole with an inner diameter less than an outerdiameter of the pipe 310 e manufactured through the forming process S3and the welding process S4 and a plug having an outer diameter with anouter diameter less than an inner diameter of the pipe 310 emanufactured through the forming process S3 and the welding process S4.

Particularly, the pipe 310 e in which the welding process S4 and/or thecutting process S5 are performed may pass through the hole formed in thedies and the plug. Here, since the bead 313 a formed on the outercircumferential surface 311 of the pipe 310 e protrudes outward from acenter of the outer circumferential surface 311 of the pipe 310 e, thebead 313 a may not pass through the hole of the dies and thus be removedwhile being plastic-deformed.

Similarly, since the bead 313 b formed on the inner circumferentialsurface 312 of the pipe 310 e protrudes toward the center of the innercircumferential surface 312 of the pipe 310 e, the bead 313 b may notpass through the plug and thus be removed while being plastic-deformed.

That is, as described above, the welded beads 313 a and 313 b formed onthe inner circumferential surface 312 and the outer circumferentialsurface 311 of the pipe 310 e may be removed through the drawing processS6. Also, since the welded bead 313 a on the inner circumferentialsurface 312 of the pipe 310 e is removed, it is possible to prevent aprotrusion from being formed on the inner circumferential surface 312 ofthe pipe 310 e when the pipe 310 e is expanded for the refrigerant pipe.

FIG. 13 illustrates a ductile stainless steel pipe in which the drawingprocess S6 is finished. That is, the beads 313 a and 313 b formed on theinner and outer circumferential surfaces 312 and 311 of the pipe 310 emay be removed through the drawing process S6.

The reason for forming the outer and inner circumferential surfaces 311and 312, which have the smooth surfaces, of the pipe 310 e is forforming the uniform inner diameter of the pipe 310 e and easilyconnecting the pipe to the other pipe. Also, the reason for forming theuniform inner diameter in the pipe 310 e is for maintaining a smoothflow of the refrigerant and a constant pressure of the refrigerant.Although not shown, after the drawing process S6, a groove (not shown)may be formed on the outer and inner circumferential surfaces 311 and312 of the pipe 310 e through mechanical working.

[Seventh Process: Bright Annealing Process S7]

The bright annealing process S7 may be understood as a process forheating the pipe 310 e from which the welded beads are removed to removeheat history and residual stress remaining in the pipe 310 e. In thisembodiment, the austenite matrix structure of about 99% or more and thedelta ferrite matrix structure of about 1% or less are formed based onthe grain area of the ductile stainless steel, and also, to increase theaverage grain size of the ductile stainless steel to about 30 μm toabout 60 μm, the heat treatment process is performed.

Particularly, the average grain diameter (or the grain size number) ofthe ductile stainless steel is a key factor in determining the lowstrength and low hardness properties of the stainless steel.Particularly, the bright annealing process S7 is performed by annealingthe pipe 310 e, from which the welded beads are removed, in a stream ofa reducing or non-oxidizing gas and cooling the annealed pipe 310 e asit is after the annealing.

As illustrated in FIG. 14, the pipe 310 e from which the welded beadsare removed passes through an annealing furnace 350 at a constant speed.The inside of the annealing furnace 350 may be filled with anatmospheric gas, and also, the inside of the annealing furnace 350 maybe heated at a high temperature by using an electric heater or a gasburner.

That is, the pipe 310 may receive a predetermined heat input whilepassing through the annealing furnace 350. Accordingly, the ductilestainless steel may have the austenite matrix structure and the averagegrain size of about 30 μm to 60 μm due to the heat input.

The heat input represents a heat amount entering into a metal member.Also, the heat input plays a very important role in metallographicmicrostructure control. Thus, in this embodiment, a heat treatmentmethod for controlling the heat input is proposed.

In the bright annealing process S7, the heat input may be determinedaccording to a heat treatment temperature, an atmospheric gas, or atransfer speed of the pipe 310 e.

In case of the bright annealing process S7 according to this embodiment,the heat treatment temperature is about 1,050° C. to about 1,100° C.,the atmospheric gas is hydrogen or nitrogen, and the transfer speed ofthe pipe 310 e is about 180 mm/min to about 220 mm/min Thus, the pipe310 e may pass through the annealing furnace 350 at a transfer speed ofabout 180 mm/min to about 220 mm/min at an annealing heat treatmenttemperature of about 1,050° C. to about 1,100° C. in the annealingfurnace 350.

Here, if the annealing heat treatment temperature is less than about1,050° C., sufficient recrystallization of the ductile stainless steeldoes not occur, the fine grain structure is not obtained, and theflattened worked structure of the grain is generated to reduce creepstrength. On the other hand, if the annealing temperature exceeds about1,100° C., high-temperature intercrystalline cracking or ductilitydeterioration may occur.

Also, when the pipe 310 e from which the welded beads are removed passesthrough the annealing furnace 350 at a transfer speed of less than about180 mm/min, the productivity is deteriorated due to a long time. On theother hand, when the pipe 310 e passes through the annealing furnace 350at a transfer speed exceeding about 220 mm/min, the stress existing inthe ductile stainless steel is not sufficiently removed, and also theaverage grain size of the austenite matrix structure is less than about30 μm. That is, if the transfer speed of the pipe 310 e is too high, theaverage grain size of the ductile stainless steel is less than about 30μm, and the low strength and low hardness properties required in thethis embodiment may not be obtained.

As described above, the ductile stainless steel pipe according to anembodiment, which is manufactured through the cold rolling process S1,the slitting process S2, the forming process S3, the welding process S4,the cutting process S5, the drawing process S6, and the bright annealingprocess S7 may be temporarily stored in a coiled state by a spool or thelike and then be shipped.

Although not shown, after the bright annealing process S7 is completed,shape correction and surface polishing processing may be furtherperformed.

<Fatigue Failure Test>

FIG. 15 is a graph illustrating result values obtained through an S-Ncurve test for comparing fatigue limits of the ductile stainless steelpipe according to an embodiment and a copper pipe according to therelated art, and FIG. 16 is a graph illustrating an S-N curve teat ofthe ductile stainless steel pipe according to an embodiment.

Referring to FIGS. 15 and 16, the ductile stainless steel pipe accordingto an embodiment has a fatigue limit (or endurance limit) of about200.52 MPa. This is a value greater by about 175 MPa (8 times) than thecopper pipe according to the related art having a fatigue limit of 25MPa. That is, the ductile stainless steel pipe may have improveddurability, reliability, life expectancy, and freedom in design whencompared to the copper pipe according to the related art. Hereinafter,effects of the ductile stainless steel pipe will be described in moredetail.

[Maximum Allowable Stress]

The ductile stainless steel pipe may be determined in maximum allowablestress value on the basis of the fatigue limit value. For example, themaximum allowable stress of the ductile stainless steel pipe may be setto about 200 MPa when the air conditioner 10 is started or stopped andmay be set to about 90 MPa when the air conditioner is in operation. Thereason in which the maximum allowable stress has a small value duringthe operation of the air conditioner may be understood as reflecting thestress due to the refrigerant flowing in the pipe in the operationstate.

The maximum allowable stress represents a maximum stress limit that maybe allowed to safely use a pipe or the like. For example, the pipe andthe like may receive external force during use, and stress may begenerated in the pipe due to the external force. Here, when the internalstress is equal to or greater than a certain critical stress valuedetermined by a factor such as a solid material, the pipe may bepermanently deformed or broken. Therefore, the maximum allowable stressmay be set to safely use the pipe.

[Fatigue Limit]

When repeated stress is applied continuously to a solid material such assteel, the solid material may be broken at stress much lower thantensile strength. This is called fatigue of the material, and a failuredue to the fatigue is called fatigue failure. The fatigue of thematerial occurs when the material undergoes a repeated load. Also, thematerial may be broken eventually when beyond a certain limit due to therepeated load. Here, an endurance limit in which the material is notbroken even under repeated load is defined as a fatigue limit endurancelimit.

[Relationship Between Fatigue Limit and S-N Curve]

An S-N curve shows the number of repetitions (N, cycles) until certainstress is repeated. In detail, the solid material is destroyed morequickly if it is subjected to repeated stress several times, and thenumber of repetitions of stress till the failure is affected by theamplitude of the applied stress. Thus, effects due to the degree ofstress and the number of repetitions of stress until the solid materialis broken may be analyzed through the S-N curve.

In the S-N curve test graph of FIGS. 15 and 16, a vertical axisrepresents a stress amplitude (Stress), and a horizontal axis representsa log value of the repetition number. Also, the S-N curve is a curvedrawn along the log value of the number of repetitions until thematerial is destroyed when the stress amplitude is applied. In general,the S-N curve of the metal material increases as the stress amplitudedecreases, the number of repetitions till the fracture increases. Also,when the stress amplitude is below a certain value, it is not destroyedeven if it repeats infinitely. Here, the stress value at which the S-Ncurve becomes horizontal represents the fatigue limit or endurance limitof the above-mentioned material.

[Fatigue Limit Limitation of Copper Pipe]

In the S-N curve of the copper pipe according to the related art, whichis based on fatigue failure test data of the copper pipe of FIG. 15according to the related art, it is seen that the fatigue limit of thecopper pipe according to the related art is about 25 MPa. That is,maximum allowable stress of the copper pipe is about 25 MPa. However, acase in which the stress of the pipe has a value of about 25 Mpa toabout 30 MPa when the air conditioner is started or stopped may occuraccording to an operation state of the air conditioner (see FIG. 18). Asa result, the copper pipe according to the related art has a limitationthat the lifetime of the pipe is shortened, and the durability isdeteriorated due to the stress value exceeding the degree of fatigue asdescribed above.

[Effect of Ductile Stainless Steel Pipe]

Referring to FIGS. 15 and 16, in the SN curve according to thisembodiment, which is based on the fatigue failure test data of theductile stainless steel pipe, the fatigue limit of the ductile stainlesssteel pipe is about 200.52 MPa, which is greater 8 times than that ofthe copper stainless steel pipe. That is, maximum allowable stress ofthe ductile stainless steel pipe is about 200 MPa.

The stress in the pipe provided in the air conditioner does not exceedthe maximum allowable stress of the ductile stainless steel pipe evenwhen considering the maximum operation load of the air conditioner.Accordingly, when the ductile stainless steel pipe is used in an airconditioner, the lifespan of the pipe may be prolonged, and thedurability and the reliability may be improved.

The ductile stainless steel pipe has a design margin of about 175 MPawhen compared to the fatigue limit of the copper pipe. In detail, theouter diameter of the ductile stainless steel pipe is the same as theouter diameter of the copper pipe according to the related art, and theinner diameter may be expanded.

That is, a minimum thickness of the ductile stainless steel pipe may beless than that of the copper pipe, and even in this case, maximumallowable stress may be greater than that of the copper pipe due to therelatively high design margin. As a result, there is an effect that thedegree of freedom in designing the ductile stainless steel pipe isimproved.

<Stress Measurement Test>

Stress more than the fatigue limit of the copper pipe according to therelated art may be generated in the pipe according to the operationconditions of the air conditioner. On the other hand, when the ductilestainless steel pipe is used in an air conditioner, the maximum stressvalue generated in the ductile stainless steel pipe does not reach thefatigue limit of the ductile stainless steel pipe. Hereinafter, thiswill be described in detail.

FIG. 17 is a view illustrating an attachment position of a stressmeasurement sensor for measuring stress of the pipe, and FIGS. 18 and 19are test data tables illustrating result values measured by the stressmeasurement sensor of FIG. 17.

In detail, FIG. 18(a) illustrates a stress measurement value of thecopper pipe according to the related art and the ductile stainless steelpipe by classifying the start, the operation, and the stop state of theair conditioner when the air conditioner operates in a standard coolingmode, and FIG. 18(b) illustrates a stress measurement value of thecopper pipe according to the related art and the ductile stainless steelpipe by classifying the start, the operation, and the stop state of theair conditioner when the air conditioner operates in a standard heatingmode.

Also, FIG. 19(a) illustrates a stress measurement value as illustratedin FIG. 4(a) when the air conditioner operates in an overload coolingmode, and FIG. 19(b) illustrates a stress measurement value in the casewhere the air conditioner operates in an overload heating mode asillustrated in FIG. 4(b).

[Installation Position of Stress Measurement Sensor]

Referring to FIG. 17, a plurality of stress measurement sensors may beinstalled in the suction pipe 210 for guiding the refrigerant to besuctioned into the compressor 100 and the discharge pipe 220 for guidingthe refrigerant compressed at a high temperature and high pressure inthe compressor to the condenser. In detail, the suction pipe 210 may beconnected to the gas/liquid separator 150 to guide the refrigerant sothat the refrigerant is suctioned into the gas/liquid separator 150.Also, the refrigerant passing through the suction pipe 210 and thedischarge pipe 220 may include the R32, the R134a, or the R401a.

In this embodiment, the R410a may be used as the refrigerant.

Since the refrigerant passing through the compressor 100 in view of theair conditioner cycle is a high-temperature high-pressure gasrefrigerant, stress acting on the discharge pipe 220 is greater thanthat acting on other refrigerant pipes.

The compressor 100 may generate vibration during the compression of thelow-pressure refrigerant into the high-pressure refrigerant. The stressof the pipes connected to the compressor 100 and the gas/liquidseparator 150 may increase due to the vibration. Therefore, since thestress in the suction pipe 210 and the discharge pipe 220 are relativelyhigher than those of the other connection pipe, a stress measurementsensor may be installed in each of the suction pipe 210 and thedischarge pipe 220 to confirm whether the stress is within the maximumallowable stress.

Also, the suction pipe 210 and the discharge pipe 220 may have thehighest stress at a bent portion. The stress measuring sensor may beinstalled in two bent portions 215 a and 215 b of the suction pipe 210and two bent portions 225 a and 225 b of the discharge pipe 220 toconfirm whether stress acting on each of the suction pipe 210 and thedischarge pipe 220 is within the maximum allowable stress.

[Stress Measurement of Copper Pipe According to Related Art]

Referring to FIGS. 18 and 19, when the suction pipe and the dischargepipe are provided as the copper pipe according to the related art, themaximum stress value is measured to about 4.9 MPa at the start, about9.6 MPa at the operation, and about 29.1 MPa at the stop. As describedabove, the maximum stress measurement value of about 29.1 MPa at thestop exceeds the maximum allowable stress value (about 25 MPa) of thecopper pipe. Thus, the durability of the pipe may be shortened toshorten the lifespan of the pipe.

[Stress Measurement of Ductile Stainless Steel Pipe]

In case in which each of the suction pipe 210 and the discharge pipe 220is provided as the ductile stainless steel pipe according to anembodiment, the stress value is measured to about 19.2 MPa at the start,about 23.2 MPa at the operation, and about 38.7 MPa at the stop. Thatis, the measured stress value in the ductile stainless steel pipesatisfies the maximum allowable stress of about 200 MPa (start/stop) orabout 90 MPa (operation) or less, and a difference from the maximumallowable stress is also very large.

Thus, the ductile stainless steel pipe has the improved durability ascompared with the copper pipe according to the related art, and when theductile stainless steel pipe is used as the suction pipe 210 and thedischarge pipe 220, it provides the improved pipe lifespan and theimproved reliability when compared to the existing copper pipe.

<Improvement of Performance (COP)>

FIG. 20 is a graph illustrating result values obtained through a testfor comparing pressure losses within the pipes when each of the ductilestainless steel pipe according to an embodiment and the copper pipeaccording to the related art is used as a gas pipe, and FIG. 21 is atest result table illustrating performance of the ductile stainlesssteel pipe according to an embodiment and the copper pipe according tothe related art. The gas pipe may be understood as a pipe for guiding aflow of an evaporated low-pressure gas refrigerant or a compressedhigh-pressure gas refrigerant on the basis of the refrigerant cycle.

In more detail, FIGS. 20(a) and 21(a) are test graphs in the standardpipe (about 5 m), and

FIGS. 20(b) and 21(b) are test graphs in the long pipe (about 50 m).

[Comparison of Pressure Loss in Pipe]

Referring to FIGS. 20(a) and 20(b), a vertical axis of the graphrepresents a pressure change amount or a pressure loss amount(ΔP=Pin-Pout, Unit KPa) in the gas pipe, and a horizontal axisrepresents the cooling mode or the heating mode of the air conditioner.

As described above, the ductile stainless steel pipe according to anembodiment is significantly improved in durability and degree of designfreedom when compared to the copper pipe according to the related art.Therefore, the ductile stainless steel pipe has the same outer diameteras the copper pipe and may have an inner diameter expanded more than thecopper pipe. The ductile stainless steel pipe may decrease in flowresistance and increase in flow rate of the refrigerant when compared tothe copper pipe due to the expanded inner diameter. Also, the ductilestainless steel pipe may be reduced in pressure loss in the pipe whencompared to the copper pipe according to the related art.

[Comparison of Pressure Loss in Standard Pipe]

Referring to FIG. 20(a), the pressure loss with the pipe of the gas pipeis formed so that the pressure loss of the ductile stainless steel pipeis less by about 2.3 KPa than that of the copper pipe according to therelated art with respect to the standard pipe having a length of about 5m. In detail, in the cooling mode, a pressure loss (ΔP) of the ductilestainless steel pipe is about 6.55 KPa, and a pressure loss (ΔP) of thecopper pipe is about 8.85 KPa. That is, in the cooling mode of thestandard pipe (about 5 m), the pressure loss of the ductile stainlesssteel pipe is less by about 26% than that of the copper pipe.

Also, the pressure loss (ΔP) of the ductile stainless steel pipe is lessby about 1.2 KPa than that (ΔP) of the copper pipe according to therelated art in the heating mode of the standard pipe (about 5 m). Thatis, in the heating mode, a pressure loss (ΔP) of the ductile stainlesssteel pipe is about 3.09 KPa, and a pressure loss (ΔP) of the copperpipe is about 4.29 KPa. That is, in the heating mode of the standardpipe (about 5 m), the pressure loss of the ductile stainless steel pipeis less by about 28% than that of the copper pipe.

[Comparison of Pressure Loss in Long Pipe]

Referring to FIG. 20(b), the pressure loss with the pipe of the gas pipeis formed so that the pressure loss of the ductile stainless steel pipeis less by about 16.9 KPa than that of the copper pipe according to therelated art with respect to the long pipe having a length of about 50 m.That is, in the cooling mode, a pressure loss (ΔP) of the ductilestainless steel pipe is about 50.7 KPa, and a pressure loss (ΔP) of thecopper pipe is about 67.6 KPa. That is, in the cooling mode of the longpipe (about 50 m), the pressure loss of the ductile stainless steel pipeis less by about 26% than that of the copper pipe.

Also, the pressure loss (ΔP) of the ductile stainless steel pipe is lessby about 10.2 KPa than that (ΔP) of the copper pipe according to therelated art in the heating mode of the long pipe (about 50 m). That is,in the heating mode, a pressure loss (ΔP) of the ductile stainless steelpipe is about 29.03 KPa, and a pressure loss (ΔP) of the copper pipe isabout 39.23 KPa. That is, in the heating mode of the long pipe (about 50m), the pressure loss of the ductile stainless steel pipe is less byabout 26% than that of the copper pipe.

[Coefficient of Performance]

A refrigerant pressure loss may occur in the gas pipe and the suctionpipe 210 or the discharge pipe 220 of the compressor 100. Therefrigerant pressure loss causes an adverse effect such as decrease inrefrigerant circulation amount, decrease in volume efficiency, increasein compressor discharge gas temperature, increase in power per unitrefrigeration capacity, and decrease in coefficient of performance(COP).

Therefore, as illustrated in FIG. 20, when the gas pipe, the suctionpipe, or the discharge pipe is provided as the ductile stainless steelpipe, the pressure loss in the pipe may be reduced when compared to thecopper pipe according to the related art, a compressor work of thecompressor (e.g., power consumption (kW)) may decrease, and thecoefficient of performance (COP) may increase.

The coefficient of performance (COP) may be a measure of the efficiencyof a mechanism for lowering or raising the temperature, such as therefrigerating machine, the air conditioner, the heat pump and may bedefined as a ratio of the output or supplied heat quantity (coolingcapacity or heating capacity) with respect to the quantity of the inputwork. Since the heat pump is a mechanism for rising a temperature, theheat pump may be called a heating performance coefficient and expressedas COPh, and the refrigerator or the air conditioner is a mechanism forlowering a temperature, the refrigerator or the air conditioner may becalled a cooling performance coefficient and expressed as COPc. Also,the coefficient of performance (COP) is defined as a value obtained bydividing the heat quantity Q extracted from a heat source or supplied tothe heat source by the work of the mechanical work.

[Comparison of Coefficient of Performance in Standard Pipe]

Referring to FIG. 21(a), the cooling capacity is about 9.36 kW for thecopper pipe and about 9.45 kW for the ductile stainless steel pipe inthe cooling mode of the standard pipe (5 m). That is, the heat quantityQ of the ductile stainless steel pipe is greater by about 100.9% thanthat of the copper pipe. Also, the power consumption is about 2.07 kWfor the copper pipe and about 2.06 kW for the ductile stainless steelpipe. Therefore, since the COP is about 4.53 in the copper pipe andabout 4.58 in the ductile stainless steel pipe, the ductile stainlesssteel pipe is improved to about 100.9% of the copper pipe according tothe related art.

Also, in the heating mode of the standard pipe (about 5 m), the heatingcapacity is about 11.28 kW for the copper pipe and about 11.31 kW forthe ductile stainless steel pipe. That is, the heat quantity Q of theductile stainless steel pipe is greater by about 100.2% than that of thecopper pipe. Also, the power consumption is about 2.55 kW for the copperpipe and about 2.55 kW for the ductile stainless steel pipe. Therefore,since the COP is about 4.43 in the copper pipe and about 4.44 in theductile stainless steel pipe, the ductile stainless steel pipe isimproved to about 100.2% of the copper pipe according to the relatedart.

[Comparison of Coefficient of Performance in Long Pipe]

The improvement of the efficiency (performance coefficient) due to thereduction of the pressure loss on the pipe is more evident in the pipe(about 50 m) than the standard pipe (about 5 m). That is, as the lengthof the pipe becomes longer, the performance of the ductile stainlesssteel pipe improved when compared to the copper pipe according to therelated art may be further improved.

Referring to FIG. 21(b), the cooling capacity is about 7.77 kW for thecopper pipe and about 8.03 kW for the ductile stainless steel pipe inthe cooling mode of the long pipe (about 50 m). That is, the heatquantity Q of the ductile stainless steel pipe is greater by about103.4% than that of the copper pipe. Also, the power consumption isabout 2.08 kW for the copper pipe and about 2.08 kW for the ductilestainless steel pipe. Therefore, since the COP is about 3.74 in thecopper pipe and about 3.86 in the ductile stainless steel pipe, theductile stainless steel pipe is improved to about 103.2% of the copperpipe according to the related art.

Also, in the heating mode of the long pipe (about 50 m), the heatingcapacity is about 8.92 kW for the copper pipe and about 9.07 kW for theductile stainless steel pipe. That is, the heat quantity Q of theductile stainless steel pipe is greater by about 101.7% than that of thecopper pipe. Also, the power consumption is about 2.54 kW for the copperpipe and about 2.53 kW for the ductile stainless steel pipe. Therefore,since the COP is about 3.51 in the copper pipe and about 3.58 in theductile stainless steel pipe, the ductile stainless steel pipe isimproved to about 102% of the copper pipe according to the related art.

<Corrosion Resistance Test>

FIG. 22 is a view illustrating a plurality of ductile stainless steelpipes, aluminum (Al) pipes, and copper pipes, which are objects to betested for corrosion resistance, FIG. 23 is a table illustrating resultsobtained by measuring a corrosion depth for each pipe in FIG. 22, andFIG. 24 is a graph illustrating results of FIG. 23.

Corrosion resistance represents a property of a material to withstandcorrosion and erosion. It is also called corrosion resistance. Ingeneral, stainless steel or titanium is more corrosion resistant thancarbon steel because it is not well corroded. The corrosion resistancetest includes a salt water spray test and a gas test. The resistance ofthe product to the atmosphere including the salt may be determinedthrough the corrosion resistance test to examine the heat resistance,the quality and uniformity of the protective coating.

[Complex Corrosion Test]

Referring to FIGS. 22 to 24, when the cyclic corrosion test is performedon the ductile stainless steel pipe according to an embodiment togetherwith comparative groups (Al, Cu) of the other pipe, it is confirmed thatthe corrosion resistance is the most excellent because the corrosiondepth (μm) is the smallest value in comparison with the comparativegroup. Hereinafter, this will be described in detail.

The cyclic corrosion test represents a corrosion test method in whichatmospheres of salt spraying, drying and wetting are repeatedlyperformed for the purpose of approaching or promoting the naturalenvironment. For example, evaluation may be carried out by setting thetest time to be 30 cycles, 60 cycles, 90 cycles, 180 cycles, and thelike, with 8 hours of one cycle, 2 hours of spraying with salt, 4 hoursof drying, and 2 hours of wetting. The salt spraying test during thecomplex corrosion test is the most widely used as an accelerated testmethod for examining the corrosion resistance of plating and is a testfor exposing a sample in the spray of saline to examine the corrosionresistance.

Referring to FIG. 22, a plurality of ductile stainless steel pipes S1,S2, and S3, a plurality of aluminum pipes A1, A2, and A3, and aplurality of copper pipes C1, C2, and C3 in which the complex corrosiontest is performed, are illustrated, and the corrosion depth (μm) wasmeasured by defining arbitrary positions D1 and D2 in each pipe.

[Test Result and Advantages of Ductile Stainless Steel Pipe]

Referring to FIGS. 23 and 24, the pip measured to have the deepestcorrosion depth is the aluminum pipe having an average of about 95 μm.Next, the average copper pipe is about 22 μm, and the ductile stainlesssteel pipe has an average value of about 19 μm, which is the mostcorrosion-resistant measurement value. Also, the maximum value Max ofthe corrosion depth μm is the deepest of aluminum pipe to about 110 μm,followed by copper pipe to about 49 μm, and the soft stainless steelpipe to about 36 μm.

Attempts have been made to use the aluminum pipe to replace the copperpipe according to the related art. However, since the corrosionresistance is low as in the above-mentioned test results, there is agreat disadvantage that the corrosion resistance is lowest. On the otherhand, the ductile stainless steel pipe has the most excellent corrosionresistance and is superior in durability and performance to the pipeaccording to the related art.

<Bending Test>

In the case of installing an air conditioner by connecting pipes to eachother according to individual installation environments, the pipe is notonly a straight pipe, but also a bent pipe formed by bending externalforce of a worker installing the pipe. Also, the straight pipe or thebent pipe connects the outdoor unit to the indoor unit.

The stainless steel pipe according to the related art has strengthgrater than that of the copper pipe. Therefore, due to the high strengthof the stainless steel pipe according to the related art, it is verydifficult for an operator to apply external force to the pipe to form abent pipe. Therefore, there has been a limitation that the copper pipeor the aluminum pipe has to be used for the convenience of installationwork.

However, the strength of the ductile stainless steel pipe according toan embodiment may be lower than that of the stainless steel pipeaccording to the related art and may be lowered to a level higher thanthat of the copper pipe according to the related art. Thus, since theabove-mentioned bent pipe or the like may be formed, the low moldabilityof the stainless steel pipe according to the related art may be solved.Hereinafter, the bending test will be described below in detail.

[Shape of Bent Pipe and Curvature Radius]

FIG. 25 is view illustrating a shape in which the ductile stainlesssteel pipe is bent according to an embodiment, FIG. 26 is across-sectional view illustrating a portion of the bent pipe, and FIG.27 is a graph illustrating results obtained through a test for comparingbending loads according to deformation lengths of the ductile stainlesssteel pipe, the copper pipe, and the aluminum pipe.

Referring to FIG. 25, the ductile stainless steel pipe according to anembodiment may be bent by bending force. For example, the ductilestainless steel pipe may have an L-shape as illustrated in FIG. 25(a) oran S-shape as illustrated in FIG. 25(b).

Referring to FIGS. 25(a) and 25(b), a central line of the ductilestainless steel pipe may include a curved portion having a curvature soas to be bent in the other direction in one direction. Also, the curvehas a curvature radius R.

The curvature radius R is defined as a value indicating a degree ofcurvature at each point of the curve. The curvature radius R of theductile stainless steel pipe forming the curved line may include aminimum curvature radius Rmin that may be used in a pipe which does notgenerate wrinkles even when the straight pipe is formed into a curvedline and does not generate vibration. Also, the minimum curvature radiusRmin may be measured in a bent pipe that meets a setting criterion for aratio of maximum and minimum outside diameters.

[Ratio of Maximum/Minimum Outer Diameters of Ductile Stainless SteelPipe]

Referring to FIG. 26, the ductile stainless steel pipe may be providedas a bent pipe so that a ratio (E/F) of a maximum outer diameter (F) toa minimum outer diameter (E) is more than 0.85 and less than 1.

The ratio of the maximum and minimum outside diameters (E/F) is aconservatively estimated standard based on the standards of ASME(American Society of Mechanical Engineers) and JIS (Japanese IndustrialStandards) (see Table 7).

Table 7 below shows setting criteria for the ratio of the maximum andminimum diameters.

TABLE 7 ASME (F − E) < 0.08 * D JIS When R > 4D, E > (2/3) * D SettingCriteria (E/F) > 0.85

In Table 7, D represents a value of the straight pipe (a referencepipe), and R represents a curvature radius.

Comparison of Bending Property of Ductile Stainless Steel Pipe, CopperPipe, and Aluminum Pipe]

FIG. 27 illustrates results of testing the bending properties of theductile stainless steel pipe satisfying the setting criteria (ratio ofmaximum and minimum outside diameters). In the bending property test,the ductile stainless steel pipe has a diameter Φ of about 15.88 mm.

The bending represents bending downward or upward in a state in whichthe beam is bent when a load is applied. When the beam is bent downward,tensile force acts on the bottom portion, and when the beam is bentupward, compressive force acts on the bottom portion.

Referring to FIG. 27, force N applied to the aluminum pipe, the copperpipe, and the ductile stainless steel pipe according to the deformationlength (mm), each of which has a pipe diameter D of about 15.88 mm isillustrated.

When the minimum curvature radius Rmin is measured at the pipe having adiameter Φ of about 15.88 mm, the copper pipe has a diameter of about 85mm, and the ductile stainless steel pipe has a diameter of about 70 mm.Accordingly, since the ductile stainless steel pipe has a curvatureradius R less than that of the copper pipe, it may be bent to be equalto or higher than that of the copper pipe.

Thus, since the ductile stainless steel pipe forms the curved pipe at alevel equivalent to that of the copper pipe, the moldability may beimproved when compared to the stainless steel pipe according to therelated art. Here, the bending force of the worker is assumed to themaximum bending load of the copper pipe and the aluminum pipe. In thisembodiment, the bending force of the worker may be about 900 N.

In the graph of the bending property test result, the force N applied inthe section of about 0 mm to about 2.5 mm of the deformation length maysharply increase, and then the force at the deformation length maygradually decrease in inclination to approach the maximum force N.

Also, in the graph of the bending property test result, the maximumbending load of the flexible stainless steel pipe may be about 750 N,and the maximum bending load of each of the copper pipe and the aluminumpipe may be about 900 N. That is, the maximum bending load of theductile stainless steel pipe is less than that of the pipe according tothe related art.

Therefore, the worker may form the ductile stainless steel pipe to bebent by using force within about 83% of the maximum bending load of eachof the copper pipe and the aluminum pipe. As a result, the worker mayform the ductile stainless steel pipe to be bent by applying force lessthan that applied to form the copper pipe and the aluminum pipe to bebent.

In summary, the ductile stainless steel pipe according to an embodimenthas an effect of improving the moldability when compared to thestainless steel pipe, the copper pipe and the aluminum pipe according tothe related art. Therefore, the easy in the installation may beimproved.

FIG. 28 is a refrigeration cycle diagram of an air conditioner to whichthe ductile stainless steel pipe is applied according to anotherembodiment.

[Refrigerant Pipe Constituted by New Material Pipe]

Referring to FIG. 28, the air conditioner 10 according to thisembodiment may have air-conditioning capacity of about 7 kW to about 8kW. The air conditioner 10 may include a refrigerant pipe 50 a guiding aflow of the refrigerant circulating through the refrigeration cycle. Therefrigerant pipe 50 a may include a new material pipe. Since the newmaterial pipe has thermal conductivity less than that of the copperpipe, when the refrigerant flows through the refrigerant pipe 50 a, aheat loss may be less than that a case in which the refrigerant flowsthrough the copper pipe.

[First Refrigerant Pipe]

In detail, the refrigerant pipe 50 a includes a first refrigerant pipe51 a extending from the second port 112 of the flow control valve 110 tothe manifold 130, i.e., the outdoor heat exchanger 120. The firstrefrigerant pipe 51 a may be provided as the new material pipe.

A high-pressure gas refrigerant flows through the first refrigerant pipe51 a during a cooling operation, and a low-pressure gas refrigerantflows during a heating operation. The first refrigerant pipe 51 a mayhave an outer diameter of about 15.80 mm to about 15.95 mm on the basisof the air-conditioning capacity of the air conditioner 10.

For example, referring to Table 4, the first refrigerant pipe 51 a hasan outer diameter of about 15.88 mm and a minimum thickness of about0.41 mm on the basis of JIS B 8607. Thus, the first refrigerant pipe 51a may have an inner diameter of about 15.06 mm (=15.88−2×0.41) or less.

[Second Refrigerant Pipe]

The refrigerant pipe 50 a further includes a second refrigerant pipe 52a extending from the distributor 140 to the main expansion device 155.The second refrigerant pipe 52 a may be provided as the new materialpipe.

A high-pressure liquid refrigerant flows through the second refrigerantpipe 52 a during the cooling operation, and a low-pressure liquidrefrigerant flows during the heating operation. The second refrigerantpipe 52 a may have an outer diameter of about 9.45 mm to about 9.60 mmon the basis of the air-conditioning capacity of the air conditioner 10.

For example, referring to Table 4, the second refrigerant pipe 52 a hasan outer diameter of about 9.52 mm and a minimum thickness of about 0.24mm on the basis of JIS B 8607. Thus, the second refrigerant pipe 52 amay have an inner diameter of about 9.04 mm (=9.52−2×0.24) or less.

[Third Refrigerant Pipe]

The refrigerant pipe 50 a further includes a third refrigerant pipe 53 aextending from the main expansion device 155 to the first service valve175. The third refrigerant pipe 53 a may be provided as the new materialpipe.

A high-pressure liquid refrigerant flows through the third refrigerantpipe 53 a during the cooling and heating operations. The thirdrefrigerant pipe 53 a may have an outer diameter of about 9.45 mm toabout 9.60 mm on the basis of the air-conditioning capacity of the airconditioner 10.

For example, referring to Table 4, the third refrigerant pipe 53 a hasan outer diameter of about 9.52 mm and a minimum thickness of about 0.24mm on the basis of JIS B 8607. Thus, the third refrigerant pipe 53 a mayhave an inner diameter of about 9.04 mm (=9.52−2×0.24) or less.

[Fourth Refrigerant Pipe]

The refrigerant pipe 50 a further includes a fourth refrigerant pipe 54a extending from the second service valve 176 to the third port 113 ofthe flow control valve 110. The fourth refrigerant pipe 54 a may beprovided as the new material pipe.

A low-pressure gas refrigerant flows through the fourth refrigerant pipe54 a during the cooling operation, and a high-pressure gas refrigerantflows during the heating operation. The fourth refrigerant pipe 54 a mayhave an outer diameter of about 15.80 mm to about 16.05 mm on the basisof the air-conditioning capacity of the air conditioner 10.

For example, referring to Table 4, the fourth refrigerant pipe 54 a hasan outer diameter of about 15.88 mm and a minimum thickness of about0.41 mm on the basis of JIS B 8607. Thus, the fourth refrigerant pipe 54a may have an inner diameter of about 15.06 mm (=15.88−2×0.41) or less.

The air conditioner having the above-described configuration may havefollowing effects.

In detail, the austenite type stainless steel pipe may be applied tosecure ductility at the level of the copper pipe when compared to thestainless steel pipe according to the related art, and thus, the bentstainless steel pipe may be applied to the refrigerant circulationcycle. That is, the degree of freedom of forming the refrigerant pipemay increase when compared to the stainless steel pipe according to therelated art. Also, the relatively inexpensive ductile stainless steelpipe may be used without using expensive copper pipe.

Also, since the ductile stainless steel pipe according to the embodimenthas the strength and the hardness greater than those of the copper pipewhile having the ductility at the level of the copper pipe, the pressureresistance may be remarkably superior to that of the copper pipe, andvarious kinds of new refrigerants having the high saturated vaporpressure may be used in the refrigerant cycle. There is an advantagethat the so-called degree of freedom of the refrigerant increases.

Also, since the stainless steel pipe having the strength and thehardness greater than those of the copper pipe has a stress margingreater than that of the copper pipe, the vibration absorptioncapability may be remarkably superior to that of the copper pipe. Thatis to say, in case of the stainless steel pipe, it is unnecessary tolengthen the pipe so as to absorb the vibration and the noise, it may beunnecessary to bend the pipe several times. Thus, it may be easy tosecure the spaced for installing the refrigerant cycle, and themanufacturing cost may be reduced by reducing the length of the pipe.

Also, since the ductility of the ductile stainless steel pipe accordingto the embodiment is improved, the workability of the pipe may increase.Also, since the ductile stainless steel pipe has corrosion resistancesuperior to that of the copper pipe, the lifespan of the pipe may beprolonged.

Also, since the suction pipe disposed adjacent to the compressor may beimproved in strength to prevent the suction pipe from being vibrated anddamaged. Also, since the ductility of the suction pipe increases, thesuction pipe may be worked (bent) and thus easily installed in thelimited space.

Also, since the suction pipe constituting the ductile stainless has thestrength greater than that of the copper pipe while securing theductility at the level of the copper pipe, the pipe may be reduced inthickness. That is, even if the pipe has a thickness less than that ofthe copper pipe, the limit pressure of the pipe may be maintained toreduce the thickness of the pipe.

Also, since the discharge pipe disposed at the discharge side of thecompressor to allow the high-pressure refrigerant to flow therethroughmay be improved in strength to prevent the discharge pipe from beingvibrated and damaged. Also, since the ductility of the discharge pipeincreases, the suction pipe may be worked (bent) and thus easilyinstalled in the limited space.

Also, since the discharge pipe constituting the ductile stainless hasthe strength greater than that of the copper pipe while securing theductility at the level of the copper pipe, the pipe may be reduced inthickness. That is, even if the pipe has a thickness less than that ofthe copper pipe, the limit pressure of the pipe may be maintained toreduce the thickness of the pipe.

As a result, the suction/discharge pipes may increase in inner diameterunder the same outer diameter as the copper pipe, and the pressure lossof the refrigerant flowing through the pipe may be reduced due to theincrease of the inner diameter. As the pressure loss within the pipedecreases, the flow rate of the refrigerant may increase to improve thecoefficient of performance (COP) of the refrigerant cycle.

Also, the outer diameter and the thickness of each of the first tofourth refrigerant pipes provided in the air conditioner may be providedwithin the optimum range to maintain the strength and the ductility ofthe pipe to the preset level or more. Therefore, the installationconvenience of the pipe may be improved.

Although embodiments have been described with reference to a number ofillustrative embodiments thereof, it should be understood that numerousother modifications and embodiments can be devised by those skilled inthe art that will fall within the scope of the principles of thisdisclosure. More particularly, various variations and modifications arepossible in the component parts and/or arrangements of the subjectcombination arrangement within the scope of the disclosure, the drawingsand the appended claims. In addition to variations and modifications inthe component parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

What is claimed is:
 1. A ductile pipe comprising: a ductile stainlesssteel pipe configured to receive a refrigerant, the ductile stainlesssteel pipe being formed of stainless steel having an austenite matrixstructure and containing a copper component, wherein, based on grainarea, at least 99% of the ductile stainless steel pipe comprises theaustenite matrix structure and 1% or less of the ductile stainless steelpipe comprises a delta ferrite matrix structure, wherein the ductilestainless steel pipe has a set outer diameter and a minimum thicknessthat is based on a design pressure of the refrigerant.
 2. The ductilepipe of claim 1, wherein the refrigerant is R410 refrigerant.
 3. Theductile pipe of claim 2, wherein the set outer diameter is 7.00 mm andthe minimum thickness is 0.18 mm.
 4. The ductile pipe of claim 2,wherein the set outer diameter is 7.94 mm and the minimum thickness is0.20 mm.
 5. The ductile pipe of claim 2, wherein the set outer diameteris 9.52 mm and the minimum thickness is 0.24 mm.
 6. The ductile pipe ofclaim 2, wherein the set outer diameter is 12.70 mm and the minimumthickness is 0.33 mm.
 7. The ductile pipe of claim 2, wherein the setouter diameter is 15.88 mm and the minimum thickness is 0.41 mm.
 8. Theductile pipe of claim 2, wherein the set outer diameter is 19.05 mm andthe minimum thickness is 0.49 mm.
 9. The ductile pipe of claim 2,wherein the set outer diameter is 22.20 mm and the minimum thickness is0.57 mm.
 10. The ductile pipe of claim 2, wherein the set outer diameteris 22.40 mm and the minimum thickness is 0.65 mm.
 11. The ductile pipeof claim 2, wherein the set outer diameter is 28.00 mm and the minimumthickness is 0.72 mm.
 12. The ductile pipe of claim 2, wherein the setouter diameter is 31.80 mm and the minimum thickness is 0.81 mm.
 13. Theductile pipe of claim 2, wherein the set outer diameter is 34.90 mm andthe minimum thickness is 0.89 mm.
 14. The ductile pipe of claim 2,wherein the set outer diameter is 38.10 mm and the minimum thickness is0.98 mm.
 15. The ductile pipe according to claim 1, wherein the designpressure is 4.15 MPa.
 16. A heat pump system having an outdoor unit andan indoor unit, wherein the outdoor unit comprises the ductile stainlesssteel pipe as claimed in claim
 1. 17. A ductile pipe, comprising: aductile stainless steel pipe configured to receive a refrigerant, theductile stainless steel pipe being formed of a stainless steel materialcomprising an austenite matrix structure, wherein, based on grain area,at least 90% of the ductile stainless steel pipe is made of theaustenite matrix structure and 1% or less of the ductile stainless steelpipe is made of a delta ferrite matrix structure, wherein the ductilestainless steel pipe has a set outer diameter and a minimum thicknessthat is determined based on a design pressure of the refrigerant. 18.The refrigeration circuit of claim 16, wherein the refrigerant is R410a.